Don't decrement an unsigned zero value below zero.

Don't decrement an unsigned zero value below zero.

In fact, don't waste time incrementing the pointer or decrementing the
count until we know that we're going to go through another trip through
the loop.  (This isn't a PDP-11 and probably isn't a VAX or 68k, so *p++
isn't going to be done by an addressing mode.)

This should fix an undefined-behavior warning.