asan report in extmatch

[Grisha Levit]

The relevant code was added in
https://git.savannah.gnu.org/cgit/bash.git/commit/?id=da43077 with
similar additions to both gmatch and extmatch, but I suspect the test
on line 912 was not meant to be in extmatch:

> .a
bash -O extglob -O dotglob -c ': ./!(.foo)'

ERROR: AddressSanitizer: heap-buffer-overflow on address 0x000102e02daf
READ of size 1 at 0x000102e02daf thread T0
    #0 extmatch sm_loop.c:912

frame #5: bash`extmatch(xc=33, s=".a", se="", p="(.foo)", pe="",
flags=161) at sm_loop.c:912:36
   909
   910   if (m1 == 0 && (flags & FNM_DOTDOT) &&
   911       (SDOT_OR_DOTDOT (s) ||
-> 912        ((flags & FNM_PATHNAME) && s[-1] == L('/') && PDOT_OR_DOTDOT(s))))
   913     return (FNM_NOMATCH);
   914
   915   /* if srest > s, we are not at start of string */