glibc: backport fix for regexec buffer read overrun

glibc: backport fix for regexec buffer read overrun

Problem found by AddressSanitizer[1]:

 Latest `grep` (git commit 1019e6e) compiled with asan may cause a
 heap-buffer-overflow when `-i` is specified.

     ./grep -i '\(\(\)*.\)*\(\)\(\)\1' /bin/chvt

 =================================================================
 ==16206==ERROR: AddressSanitizer: heap-buffer-overflow on address

1. https://debbugs.gnu.org/34140

Ref: https://sourceware.org/bugzilla/show_bug.cgi?id=24114
Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
[commit title and description facelift]
Signed-off-by: Petr Štetiar <ynezz@true.cz>