perl #122747: localize PL_curpm to null in _core_swash_init

Set PL_curpm to null before we do any swash intialization
in _core_swash_init(). This "hides" the current regop from the
swash code, with the intent of prevent weird reentrancy bugs
when the swashes are initialized.

Long term you could argue that we should just not use the regex
engine to initialize a swash, and then this would be unnecessary.

Thanks to FC for the suggestion!