Fix XPath freeing error.

	Bug: 5533654

Applied a patch from Chrome source code to prevent potential
multiple freeing and memory corruption in XPath. This patch
will become unnecessary once we pull in new libxml2.
The Chrome changes are here:
http://codereview.chromium.org/5196003
http://codereview.chromium.org/7508039



Change-Id: I0dd573e2f8e3cfbd1290735e68e44ebb13597482
Signed-off-by: Selim Gurun <sgurun@google.com>