net: dns: dispatcher: fix OOB array access

Validate that the file descriptor is not a negative number before
writing to the `dispatch_table` `ctx` field. Setting file descriptors
to `-1` is the standard "not in use" value, and in fact the entire array
of `fds` is set to this value in `dns_resolve_init_locked`. This
resolves memory corruption of whichever variable is unfortunate to exist
just before `dispatch_table` in memory.

Signed-off-by: Jordan Yates <jordan@embeint.com>

Author: JordanYates

subsys/net/lib/dns/dispatcher.c

@@ -345,6 +345,10 @@ int dns_dispatcher_unregister(struct dns_socket_dispatcher *ctx)
 			goto out;
 		}
 
+		if (ctx->fds[i].fd < 0) {
+			continue;
+		}
+
 		dispatch_table[ctx->fds[i].fd].ctx = NULL;
 	}