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

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

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

Committed: https://code.google.com/p/google-breakpad/source/detail?r=1179

[mseaborn]

Although this is in third_party, fixes seem to be accepted, based on the version
history.

[Mark Mentovai]

LGTM

[mseaborn]

Committed patchset #1 manually as r1179 (presubmit successful).