Fix crash with empty DHCP SNAME option.

dhcp-option=66,

sets the servername to nothing, and causes a segfault at DHCP
transaction time. There may be other ways to provoke this, for
instance by using an empty filename.

The patch make safe_strncpy() even safer, by handing a NULL src argument.

Thanks to Jeff Allen for spotting and reporting this.

src/util.c

@@ -327,13 +327,15 @@ void *safe_malloc(size_t size)
 }
 
 /* Ensure limited size string is always terminated.
- * Can be replaced by (void)strlcpy() on some platforms */
+   Can be replaced by (void)strlcpy() on some platforms.
+   src may be NULL in which case we return an empty string. */
 void safe_strncpy(char *dest, const char *src, size_t size)
 {
   if (size != 0)
     {
-      dest[size-1] = '\0';
-      strncpy(dest, src, size-1);
+      dest[0] = dest[size-1] = '\0';
+      if (src)
+	strncpy(dest, src, size-1);
     }
 }