Fixing a clang warning.

This is a followup change from https://breakpad.appspot.com/569002/

This prevents push_back from ever calling Realloc()
with 0 (which could happen if wasteful_vector was
constructed with size_hint set to 0, causing
allocated_ to be 0.
Review URL: https://breakpad.appspot.com/576002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1166 4c0a9323-5329-0410-9bdc-e9ce6186880e
1 file changed