SIGSEGV in patmatch() -- Fedora bug #1245116

[Kamil Dudka <kdudka@xxxxxxxxxx>]

A user of Fedora reported a crash of zsh-5.0.8 in patmatch():

https://bugzilla.redhat.com/1245116

The automatically recorded backtrace:

https://bugzilla.redhat.com/attachment.cgi?id=1054236

... shows that 'chrop' and 'chrend' pointed off the allocated memory
at this line:

http://repo.or.cz/w/zsh/mirror.git/blob/a0862f63:/Src/pattern.c#l2541

My guess is that the invalid addresses were loaded from 'exactpos' and 
'exactend' but static variables are not captured in the backtrace to
actually confirm it.

Unfortunately, I was not successful in reproducing the crash locally,
neither the original reporter was able to trigger it on demand.  Hence,
I am sharing it here in case anybody recognized the actual cause just
from the backtrace.

Kamil