libpng error handlers must not return to their callers, thus longjmp out.

* io-png.c (png_simple_error_callback): libpng error handlers must
	not return to their callers, thus longjmp out. This prevents
	annoying stderr output from the default error handler.

gdk-pixbuf/io-png.c

@@ -183,6 +183,8 @@ png_simple_error_callback(png_structp png_save_ptr,
                              _("Fatal error in PNG image file: %s"),
                              error_msg);
         }
+
+        longjmp (png_save_ptr->jmpbuf, 1);
 }
 
 static void