Fixing a clang warning.

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.
Committed: https://code.google.com/p/google-breakpad/source/detail?r=1166

[Ivan Penkov]

.

[Mark Mentovai]

LGTM