GTK: add NULL check in StatusBubbleGtk to fix crasher.
GTK: add NULL check in StatusBubbleGtk to fix crasher.

I couldn't actually repro the crash and after inspecting the code, I'm not sure why it's occuring. The only possibility seems that the status bubble widget is getting unparented at some point (probably shutdown). This check matches checks elsewhere in StatusBubbleGtk.

BUG=44971
TEST=none

Review URL: http://codereview.chromium.org/2199001

git-svn-id: http://src.chromium.org/svn/trunk/src@48230 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 940b7c7c696f582eca1fd77e4a1a734dabf6e74a
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser