Fix a "set but not used" compiler warning in libdisasm

BUG=none
TEST=none apart from compiling with "-Wall -Werror"

Review URL: https://breakpad.appspot.com/594002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1179 4c0a9323-5329-0410-9bdc-e9ce6186880e