Drive by leak fix when code spelunking

_cogl_shader_compile_real spews a warning when
shader compilation fails if COGL_GL_DEBUG is
defined.  This warning is never freed.

https://bugzilla.gnome.org/show_bug.cgi?id=672243

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-shader.c

@@ -384,6 +384,7 @@ _cogl_shader_compile_real (CoglHandle handle,
           g_warning ("Failed to compile GLSL program:\nsrc:\n%s\nerror:\n%s\n",
                      shader->source,
                      log);
+          g_free (log);
         }
 #endif
     }