Bug 793951 - Fix crashes when some external APIs fail

Check the return values of some functions and set errors or print
a message to stderr if they fail.

app/display/gimpdisplayshell-layer-select.c

@@ -75,9 +75,10 @@ gimp_display_shell_layer_select_init (GimpDisplayShell *shell,
                                       gint              move,
                                       guint32           time)
 {
-  LayerSelect *layer_select;
-  GimpImage   *image;
-  GimpLayer   *layer;
+  LayerSelect   *layer_select;
+  GimpImage     *image;
+  GimpLayer     *layer;
+  GdkGrabStatus  status;
 
   g_return_if_fail (GIMP_IS_DISPLAY_SHELL (shell));
 
@@ -97,7 +98,9 @@ gimp_display_shell_layer_select_init (GimpDisplayShell *shell,
 
   gtk_widget_show (layer_select->window);
 
-  gdk_keyboard_grab (gtk_widget_get_window (layer_select->window), FALSE, time);
+  status = gdk_keyboard_grab (gtk_widget_get_window (layer_select->window), FALSE, time);
+  if (status != GDK_GRAB_SUCCESS)
+    g_printerr ("gdk_keyboard_grab failed with status %d\n", status);
 }
 
 

plug-ins/common/file-png.c

@@ -888,6 +888,13 @@ load_image (const gchar  *filename,
     }
 
   info = png_create_info_struct (pp);
+  if (! info)
+    {
+      g_set_error (error, 0, 0,
+                   _("Error while reading '%s'. Could not create PNG header info structure."),
+                   gimp_filename_to_utf8 (filename));
+      return -1;
+    }
 
   if (setjmp (png_jmpbuf (pp)))
     {
@@ -1537,6 +1544,13 @@ save_image (const gchar  *filename,
     }
 
   info = png_create_info_struct (pp);
+  if (! info)
+    {
+      g_set_error (error, 0, 0,
+                   _("Error while exporting '%s'. Could not create PNG header info structure."),
+                   gimp_filename_to_utf8 (filename));
+      return FALSE;
+    }
 
   if (setjmp (png_jmpbuf (pp)))
     {

plug-ins/common/file-xpm.c

@@ -408,6 +408,9 @@ parse_colors (XpmImage  *xpm_image)
 #ifndef XPM_NO_X
   /* open the display and get the default color map */
   display  = XOpenDisplay (NULL);
+  if (display == NULL)
+    g_printerr ("Could not open display\n");
+
   colormap = DefaultColormap (display, DefaultScreen (display));
 #endif