Fix possible SEGV when no servers defined.

If there exists a --address=/<domain>/  or --server=/<domain>/#
configuration but no upstream server config unqualified by
domain then when a query which doesnt match the domain is
recieved it will use the qualfied server config and in the process
possibly make an out-of-bounds memory access.

Thanks to Daniel Danzberger for spotting the bug.

src/domain-match.c

@@ -253,9 +253,10 @@ int lookup_domain(char *domain, int flags, int *lowout, int *highout)
   if (highout)
     *highout = nhigh;
 
-  if (nlow == nhigh)
+  /* qlen == -1 when we failed to match even an empty query, if there are no default servers. */
+  if (nlow == nhigh || qlen == -1)
     return 0;
-
+  
   return 1;
 }