VectorImpl.cpp: fix benign multiplication overflow

j is a ssize_t, which can go negative. If it goes negative,
the resulting multiplication of mItemSize*j doesn't make
any sense. Since the value is never used, just don't perform
the calculation if j < 0.

Bug: 23607865
Change-Id: I14f6f6506645d582f7d67a2e2d60ead3cb18b957