Add missing free for the allocated memory from vasprintf().

The XMLDocumentParser::error() returns without deleting the
allocated memory from vasprintf() and this can make memory leak.
To prevent the memory leak, free the memory before return.

BUG=21347002

Review URL: https://chromiumcodereview.appspot.com/21347002

git-svn-id: svn://svn.chromium.org/blink/trunk@155304 bbb929c8-8fbe-4397-9dbb-9b2b20218538