re_intuit_start(): fix SEGV in /^../m code

RT #121248

Part of my recent refactoring of the code in re_intuit_start() - the bit
that searches for \n's in /^../m patterns (d0880ea) - introduced a
segfault, due to the count arg of memchr() going negative.

This is due to the stclass code (which I haven't refactored yet)
incrementing rx_origin beyond the last point it could match, then jumping
back to the MBOL block. In theory the stclass block should be fixed, but
until I get round to that, I've just added a test to the MBOL block
instead.