regmatch(): do nextchr=*locinput at top of loop

regmatch(): do nextchr=*locinput at top of loop

Currently each branch in the main regmatch() loop is responsible
re-initialising nextchar to UCHARAT(locinput) if locinput is modified.

By adding
    nextchr = UCHARAT(locinput);
to the head of the loop, we can remove most of the nextchar assignments
in the individual branches. We lose slightly for the zero-width assertions
like \b which will re-read the same nextchar, but this will make it
easier to handle non-null-terminated strings.