TypeHelpers.h: Don't underflow unsigned int

When decrementing "n", eventually n will equal zero. When that
happens, n-- 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: I9705be6580d61a164ef5fb1ec77e98a69d888438