Minor cleanups relating to size_t being unsigned

Minor cleanups relating to size_t being unsigned

In order to use -1 as a sentinel value, we should explicitly cast
to make it clear what we are doing.  It might be better to use
a less convoluted sentinel value such as SIZE_T_MAX, though.

Additionally, since size_t is unsigned and at least as wide as
an int, a loop with int index variable that compares against a
size_t for its termination check could become an infinite loop.
Make the loop index size_t for consistency.