[nrf fromtree] net: dns: dispatcher: Don't use uninitialized entries

In case CONFIG_DNS_RESOLVER_MAX_SERVERS is larger than the actual number
of DNS servers configured, some server entries may be left
uninitialized. The dispatcher needs to take this into account, otherwise
it may cause memory corruptions.

Signed-off-by: Robert Lubos <[email protected]>
(cherry picked from commit 5c6b003)

Author: rlubos

subsys/net/lib/dns/dispatcher.c

@@ -240,6 +240,10 @@ int dns_dispatcher_register(struct dns_socket_dispatcher *ctx)
 				goto out;
 			}
 
+			if (ctx->fds[i].fd < 0) {
+				continue;
+			}
+
 			if (dispatch_table[ctx->fds[i].fd].ctx == NULL) {
 				dispatch_table[ctx->fds[i].fd].ctx = ctx;
 			}
@@ -277,6 +281,10 @@ int dns_dispatcher_register(struct dns_socket_dispatcher *ctx)
 			goto out;
 		}
 
+		if (ctx->fds[i].fd < 0) {
+			continue;
+		}
+
 		if (dispatch_table[ctx->fds[i].fd].ctx == NULL) {
 			dispatch_table[ctx->fds[i].fd].ctx = ctx;
 		}