st: Fix offscreen leak if cogl_framebuffer_allocate fails

If cogl_framebuffer_allocate fails in _st_create_shadow_pipeline_from_actor, the
CoglOffscreen* that was allocated earlier in the function is leaked.

https://bugzilla.gnome.org/show_bug.cgi?id=735705

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>

src/st/st-private.c

@@ -442,6 +442,7 @@ _st_create_shadow_pipeline_from_actor (StShadow     *shadow_spec,
       if (!cogl_framebuffer_allocate (fb, &catch_error))
         {
           cogl_error_free (catch_error);
+          cogl_object_unref (offscreen);
           cogl_object_unref (buffer);
           return NULL;
         }