Fix coverity warnings on dbus

Error: CLANG_WARNING: [#def30]
dnsmasq-2.86test7/src/dbus.c:117:3: warning[deadcode.DeadStores]: Value stored to 'w' is never read
 #  115|     daemon->watches = w;
 #  116|
 #  117|->   w = data; /* no warning */
 #  118|     return TRUE;
 #  119|   }

Error: CLANG_WARNING: [#def31]
dnsmasq-2.86test7/src/dbus.c:137:3: warning[deadcode.DeadStores]: Value stored to 'w' is never read
 #  135|       }
 #  136|
 #  137|->   w = data; /* no warning */
 #  138|   }
 #  139|

Error: CHECKED_RETURN (CWE-252): [#def32]
dnsmasq-2.86test7/src/dbus.c:146: check_return: Calling "dbus_message_iter_init" without checking return value (as is done elsewhere 4 out of 5 times).
dnsmasq-2.86test7/src/dbus.c:460: example_checked: Example 1: "dbus_message_iter_init(message, &iter)" has its value checked in "dbus_message_iter_init(message, &iter)".
dnsmasq-2.86test7/src/dbus.c:573: example_checked: Example 2: "dbus_message_iter_init(message, &iter)" has its value checked in "dbus_message_iter_init(message, &iter)".
dnsmasq-2.86test7/src/dbus.c:257: example_checked: Example 3: "dbus_message_iter_init(message, &iter)" has its value checked in "dbus_message_iter_init(message, &iter)".
dnsmasq-2.86test7/src/dbus.c:427: example_checked: Example 4: "dbus_message_iter_init(message, &iter)" has its value checked in "dbus_message_iter_init(message, &iter)".
 #  144|     char *domain;
 #  145|
 #  146|->   dbus_message_iter_init(message, &iter);
 #  147|
 #  148|     mark_servers(SERV_FROM_DBUS);

Error: NEGATIVE_RETURNS (CWE-394): [#def33]
dnsmasq-2.86test7/src/dbus.c:547: negative_return_fn: Function "parse_hex((char *)hwaddr, dhcp_chaddr, 16, NULL, &hw_type)" returns a negative number.
dnsmasq-2.86test7/src/dbus.c:547: assign: Assigning: "hw_len" = "parse_hex((char *)hwaddr, dhcp_chaddr, 16, NULL, &hw_type)".
dnsmasq-2.86test7/src/dbus.c:551: negative_returns: "hw_len" is passed to a parameter that cannot be negative.
 #  549|       hw_type = ARPHRD_ETHER;
 #  550|
 #  551|->   lease_set_hwaddr(lease, dhcp_chaddr, clid, hw_len, hw_type,
 #  552|                      clid_len, now, 0);
 #  553|     lease_set_expires(lease, expires, now);

Error: CLANG_WARNING: [#def34]
dnsmasq-2.86test7/src/dbus.c:722:3: warning[deadcode.DeadStores]: Value stored to 'method' is never read
 #  720|       clear_cache_and_reload(dnsmasq_time());
 #  721|
 #  722|->   method = user_data; /* no warning */
 #  723|
 #  724|     /* If no reply or no error, return nothing */

src/dbus.c

@@ -114,7 +114,7 @@ static dbus_bool_t add_watch(DBusWatch *watch, void *data)
   w->next = daemon->watches;
   daemon->watches = w;
 
-  w = data; /* no warning */
+  (void)data; /* no warning */
   return TRUE;
 }
 
@@ -134,16 +134,20 @@ static void remove_watch(DBusWatch *watch, void *data)
 	up = &(w->next);
     }
 
-  w = data; /* no warning */
+  (void)data; /* no warning */
 }
 
-static void dbus_read_servers(DBusMessage *message)
+static DBusMessage* dbus_read_servers(DBusMessage *message)
 {
   DBusMessageIter iter;
   union  mysockaddr addr, source_addr;
   char *domain;
   
-  dbus_message_iter_init(message, &iter);
+  if (!dbus_message_iter_init(message, &iter))
+    {
+      return dbus_message_new_error(message, DBUS_ERROR_INVALID_ARGS,
+                                    "Failed to initialize dbus message iter");
+    }
 
   mark_servers(SERV_FROM_DBUS);
   
@@ -222,6 +226,7 @@ static void dbus_read_servers(DBusMessage *message)
    
   /* unlink and free anything still marked. */
   cleanup_servers();
+  return NULL;
 }
 
 #ifdef HAVE_LOOP
@@ -545,6 +550,10 @@ static DBusMessage *dbus_add_lease(DBusMessage* message)
 					 "Invalid IP address '%s'", ipaddr);
    
   hw_len = parse_hex((char*)hwaddr, dhcp_chaddr, DHCP_CHADDR_MAX, NULL, &hw_type);
+  if (hw_len < 0)
+    return dbus_message_new_error_printf(message, DBUS_ERROR_INVALID_ARGS,
+					 "Invalid HW address '%s'", hwaddr);
+
   if (hw_type == 0 && hw_len != 0)
     hw_type = ARPHRD_ETHER;
   
@@ -668,7 +677,7 @@ DBusHandlerResult message_handler(DBusConnection *connection,
 #endif
   else if (strcmp(method, "SetServers") == 0)
     {
-      dbus_read_servers(message);
+      reply = dbus_read_servers(message);
       new_servers = 1;
     }
   else if (strcmp(method, "SetServersEx") == 0)
@@ -719,7 +728,7 @@ DBusHandlerResult message_handler(DBusConnection *connection,
   if (clear_cache)
     clear_cache_and_reload(dnsmasq_time());
   
-  method = user_data; /* no warning */
+  (void)user_data; /* no warning */
 
   /* If no reply or no error, return nothing */
   if (!reply)