compositor-xrender: Fix memory leak

Fixes Clang static analyzer warning:

compositor/compositor-xrender.c:587:10: warning: Potential leak of memory pointed to by 'data'
  return ximage;
         ^~~~~~

src/compositor/compositor-xrender.c

@@ -584,6 +584,8 @@ make_shadow (MetaDisplay   *display,
        }
    }

  ximage->data = (char *) data;

  return ximage;
}