patch 8.1.1219: not checking for NULL return from alloc()

Problem:    Not checking for NULL return from alloc().
Solution:   Add checks. (Martin Kunev, closes #4303, closes #4174)
diff --git a/src/gui_gtk_f.c b/src/gui_gtk_f.c
index 6146fe6..1bff14b 100644
--- a/src/gui_gtk_f.c
+++ b/src/gui_gtk_f.c
@@ -130,6 +130,8 @@
 
     /* LINTED: avoid warning: conversion to 'unsigned long' */
     child = g_new(GtkFormChild, 1);
+    if (child == NULL)
+	return;
 
     child->widget = child_widget;
     child->window = NULL;