emcute: never return from receive loop

Without this change an attacker would be able to stop the emcute server
by sending a crafted packet triggering this branch. The solution is
using `continue` instead of `return`.

sys/net/application_layer/emcute/emcute.c

@@ -515,7 +515,7 @@ void emcute_run(uint16_t port, const char *id)
 
         if ((len < 0) && (len != -ETIMEDOUT)) {
             LOG_ERROR("[emcute] error while receiving UDP packet\n");
-            return;
+            continue;
         }
 
         if (len >= 2) {