Fix rare undefined behaviors in replace-match

Fix rare undefined behaviors in replace-match

* src/search.c (Freplace_match): Simplify by caching search_regs
components.  Fix sanity check for out-of-range subscripts;
it incorrectly allowed negative subscripts, subscripts
equal to search_regs.num_regs, and it had undefined
behavior for subscripts outside ptrdiff_t range.
Improve wording of newly-introduced replace-match diagnostic.
Rework use of opoint, to avoid setting point to an out-of-range value
in rare cases involving modification hooks.