[gnome-shell] st: Fix offscreen leak if cogl_framebuffer_allocate fails

[Ray Strode <halfline src gnome org>]

commit 941513b280433c269491572b7b1de2a7c5bab21b
Author: Aaron Plattner <aplattner nvidia com>
Date:   Fri Mar 18 16:00:05 2016 -0700

    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 |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/st/st-private.c b/src/st/st-private.c
index 559336c..d40eceb 100644
--- a/src/st/st-private.c
+++ b/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;
         }