don't trigger an integer underflow when decrementing.

When decrementing "i", eventually i will equal zero. When that
happens, i-- underflows. This causes a crash when code which uses
clang's -fsanitize=unsigned-integer-overflow is run.

Avoid trigging an unsigned integer underflow.

Change-Id: I61709cb01f56fdb36d631aa95579e8bd09cafd12