Add missing free for the allocated memory from vasprintf().
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@155304 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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