Fix error checking in mysqlnd

Closes GH-11925

NEWS

@@ -42,6 +42,8 @@ PHP                                                                        NEWS
   . Fixed bug GH-11550 (MySQL Statement has a empty query result when
     the response field has changed, also Segmentation fault).
     (Yurunsoft)
+  . Fixed invalid error message "Malformed packet" when connection is dropped.
+    (Kamil Tekiela)
 
 - Opcache:
   . Fixed bug GH-11715 (opcache.interned_strings_buffer either has no effect or

ext/mysqlnd/mysqlnd_protocol_frame_codec.c

@@ -202,6 +202,7 @@ MYSQLND_METHOD(mysqlnd_pfc, send)(MYSQLND_PFC * const pfc, MYSQLND_VIO * const v
 	if (bytes_sent <= 0) {
 		DBG_ERR_FMT("Can't %zu send bytes", count);
 		SET_CLIENT_ERROR(error_info, CR_SERVER_GONE_ERROR, UNKNOWN_SQLSTATE, mysqlnd_server_gone);
+		bytes_sent = 0; // the return type is unsigned and 0 represents an error condition
 	}
 	DBG_RETURN(bytes_sent);
 }