fix a signed/unsigned comparison warning

The comparison was farther down; we'd compare a size_t vector index
with an int index.

Complaint from clang:

vr_hwc.cpp:152:11: warning: comparison of integers of different signs:
'size_t' (aka 'unsigned long') and 'int'

Bug: None
Test: mma shows no more warning.
Change-Id: Icabb5d4bb3c554ffe01a25af975a05bd58f336a1