Ticket #4222: fix segfault in search.

Ticket #4222: fix segfault in search.

(mc_search__g_regex_match_full_safe): fix out of bound read:
g_utf8_get_char_validated() expects a nul-terminated string.

Test case: search for "test" in
https://mirrors.edge.kernel.org/pub/linux/kernel/firmware/linux-firmware-20201218.tar.xz

Found by clang-11

==10142==ERROR: AddressSanitizer: SEGV on unknown address 0x60c001e00000 (pc 0x7ffb352111c0 bp 0x7ffcb5745150 sp 0x7ffcb57450e8 T0)
==10142==The signal is caused by a READ memory access.
    #0 0x7ffb352111c0 in g_utf8_get_char_validated (/usr/lib64/libglib-2.0.so.0+0x811c0)
    #1 0x851e6d in mc_search__g_regex_match_full_safe /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/search/regex.c:297:22
    #2 0x851824 in mc_search__regex_found_cond_one /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/search/regex.c:328:10
    #3 0x84b955 in mc_search__regex_found_cond /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/search/regex.c:377:13
    #4 0x84aa07 in mc_search__run_regex /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/search/regex.c:955:17
    #5 0x848969 in mc_search__run_normal /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/search/normal.c:104:12
    #6 0x77270c in mc_search_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/search/search.c:308:15
    #7 0x514fd9 in search_content /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/find.c:1123:20
    #8 0x511917 in do_search /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/find.c:1405:26
    #9 0x512028 in find_callback /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/find.c:1597:9
    #10 0x7e285f in send_message /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/../../lib/widget/widget-common.h:243:15
    #11 0x7e36d8 in frontend_dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:307:17
    #12 0x7e34c5 in dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:583:5
    #13 0x510aa1 in run_process /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/find.c:1755:11
    #14 0x50c29e in do_find /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/find.c:1789:20
    #15 0x5098e7 in find_cmd /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/find.c:1917:17
    #16 0x504735 in midnight_execute_cmd /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/filemanager.c:1251:9
    #17 0x502e00 in midnight_callback /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/filemanager.c:1604:21
    #18 0x7eaa5c in send_message /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/../../lib/widget/widget-common.h:243:15
    #19 0x7e7ae9 in group_handle_key /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/group.c:442:19
    #20 0x7e6a4f in group_default_callback /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/group.c:567:16
    #21 0x7e31bf in dlg_key_event /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:251:19
    #22 0x7e2c12 in dlg_process_event /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:549:9
    #23 0x7e3746 in frontend_dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:320:9
    #24 0x7e34c5 in dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:583:5
    #25 0x502866 in do_nc /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/filemanager.c:1838:16
    #26 0x4ceb15 in main /tmp/portage/app-misc/mc-9999/work/mc-9999/src/main.c:455:21
    #27 0x7ffb34f16b49 in __libc_start_main (/lib64/libc.so.6+0x23b49)
    #28 0x4230f9 in _start (/tmp/portage/app-misc/mc-9999/work/mc-9999/src/mc+0x4230f9)

Signed-off-by: Andreas Mohr <and@gmx.li>
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>