cogl-framebuffer: Clear the renderbuffer list on failure

When try_creating_fbo fails it deletes any intermediate render buffers
that were created. However it doesn't clear the list so I think if it
failed a second time it would try to delete the render buffers
again. This could potentially cause problems if a subsequent fbo is
created because the destructor for the original might delete the
renderbuffers of the new fbo.

cogl/cogl-framebuffer.c

@@ -432,6 +432,10 @@ try_creating_fbo (CoglOffscreen *offscreen,
           GLuint renderbuffer = GPOINTER_TO_UINT (l->data);
           GE (glDeleteRenderbuffers (1, &renderbuffer));
         }
+
+      g_slist_free (offscreen->renderbuffers);
+      offscreen->renderbuffers = NULL;
+
       return FALSE;
     }