ui/gtk-gl-area: Plug memleak in gd_gl_area_create_context()

Receiving error in local variable err, and forgot to free it.
This patch check the return value of 'gdk_window_create_gl_context'
and 'gdk_gl_context_realize', then free err to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Message-Id: <20200831134315.1221-6-pannengyuan@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/gtk-gl-area.c

@@ -147,10 +147,21 @@ QEMUGLContext gd_gl_area_create_context(DisplayChangeListener *dcl,
     gtk_gl_area_make_current(GTK_GL_AREA(vc->gfx.drawing_area));
     window = gtk_widget_get_window(vc->gfx.drawing_area);
     ctx = gdk_window_create_gl_context(window, &err);
+    if (err) {
+        g_printerr("Create gdk gl context failed: %s\n", err->message);
+        g_error_free(err);
+        return NULL;
+    }
     gdk_gl_context_set_required_version(ctx,
                                         params->major_ver,
                                         params->minor_ver);
     gdk_gl_context_realize(ctx, &err);
+    if (err) {
+        g_printerr("Realize gdk gl context failed: %s\n", err->message);
+        g_error_free(err);
+        g_clear_object(&ctx);
+        return NULL;
+    }
     return ctx;
 }