[perl #59734] Segfault when using (?|) in regexp.

The commit obtained by bisecting isn't the source of this, it just
merely made it more visible on some architectures. For example, I can't
reproduce the crash on my 64-bits system, but valgrind catches the error.

This happens because when there's only one branch in the (?| ... ), the
value of RExC_npar is reset unconditionnaly. The attached patch fixes
the issue. 

• [perl #59734] Segfault when using (?|) in regexp. by Vincent Pit via RT
• Re: [perl #59734] Segfault when using (?|) in regexp. by demerphq
• thoughts about bug bounties (was Re: [perl #59734] Segfault when using (?|) in regexp.) by Nicholas Clark
• Re: thoughts about bug bounties (was Re: [perl #59734] Segfaultwhen using (?|) in regexp.) by Dave Mitchell
• Re: thoughts about bug bounties (was Re: [perl #59734] Segfault whenusing (?|) in regexp.) by Vincent Pit
• Re: thoughts about bug bounties (was Re: [perl #59734] Segfault when using (?|) in regexp.) by David Golden