glslstage: properly return an error when we could not create the shader stage

While it was erroring out correctly later, there were GLib warnings about
setting a GError over the top of another GError.

gst-libs/gst/gl/gstglslstage.c

@@ -436,6 +436,7 @@ _compile_shader (GstGLContext * context, struct compile *data)
   if (!_ensure_shader (data->stage)) {
     g_set_error (data->error, GST_GLSL_ERROR, GST_GLSL_ERROR_COMPILE,
         "Failed to create shader object");
+    data->result = FALSE;
     return;
   }