resource.c: Fix memory leak in coap_add_observer()

If no observer exists, a new cache key is created and storage for a
new subscription may be allocated. If this fails, the newly created
cache key must be released.

This fixes CID 1504918.

src/resource.c

@@ -759,6 +759,7 @@ static const uint16_t cache_ignore_options[] = { COAP_OPTION_ETAG };
   s = COAP_MALLOC_TYPE(subscription);
 
   if (!s) {
+    coap_delete_cache_key(cache_key);
     return NULL;
   }