Fix UMR and potential crash in Mac dump_syms.

Patch by Robert Sesek <rsesek@chromium.org>

BUG=https://code.google.com/p/google-breakpad/issues/detail?id=614
R=mark@chromium.org

Review URL: https://breakpad.appspot.com/9724002/


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1401 4c0a9323-5329-0410-9bdc-e9ce6186880e