Ticket #3579: (edit_get_bracket): fix buffer overflow.

Testcase:
create file with byte 0x28 and byte 0x00 -> open in mcedit -> move cursors

Found by Clang/AddressSanitizer

==3716==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000068886a at pc 0x000000598c09 bp 0x7ffeed4b0350 sp 0x7ffeed4b0348
READ of size 1 at 0x00000068886a thread T0
    #0 0x598c08 in edit_get_bracket /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/edit.c:1525:9
    #1 0x58ea18 in edit_find_bracket /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/edit.c:3173:32
    #2 0x596ed4 in edit_execute_cmd /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/edit.c:3960:5
    #3 0x58ed08 in edit_execute_key_command /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/edit.c:3224:5
    #4 0x5c4dcd in edit_callback /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/editwidget.c:1136:17
    #5 0x7f80f324ca11 in send_message /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/../../lib/widget/widget-common.h:167:15
    #6 0x7f80f324ca11 in dlg_key_event /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:515
    #7 0x7f80f324ca11 in dlg_process_event /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:1236
    #8 0x7f80f324e0b7 in frontend_dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:570:9
    #9 0x7f80f324cc35 in dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:1267:5
    #10 0x5c1719 in edit_files /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/editwidget.c:1255:9
    #11 0x5c0426 in edit_file /tmp/portage/app-misc/mc-9999/work/mc-9999/src/editor/editwidget.c:1194:10
    #12 0x5f048b in edit_file_at_line /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/cmd.c:704:9
    #13 0x52bf9c in do_edit /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/cmd.c:168:5
    #14 0x52bf9c in edit_cmd /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/cmd.c:742
    #15 0x52bf9c in midnight_execute_cmd /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/midnight.c:1175
    #16 0x7f80f3248019 in buttonbar_callback /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/buttonbar.c:172:42
    #17 0x7f80f324c915 in send_message /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/../../lib/widget/widget-common.h:167:15
    #18 0x7f80f324c915 in dlg_try_hotkey /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:464
    #19 0x7f80f324c915 in dlg_key_event /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:509
    #20 0x7f80f324c915 in dlg_process_event /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:1236
    #21 0x7f80f324e0b7 in frontend_dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:570:9
    #22 0x7f80f324cc35 in dlg_run /tmp/portage/app-misc/mc-9999/work/mc-9999/lib/widget/dialog.c:1267:5
    #23 0x4fc967 in create_panels_and_run_mc /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/midnight.c:954:5
    #24 0x4fc967 in do_nc /tmp/portage/app-misc/mc-9999/work/mc-9999/src/filemanager/midnight.c:1757
    #25 0x4fc967 in main /tmp/portage/app-misc/mc-9999/work/mc-9999/src/main.c:463
    #26 0x7f80f17c2953 in __libc_start_main (/lib64/libc.so.6+0x20953)
    #27 0x427008 in _start (/usr/bin/mc+0x427008)

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

src/editor/edit.c

@@ -1520,7 +1520,7 @@ edit_get_bracket (WEdit * edit, gboolean in_screen, unsigned long furthest_brack
     if (!furthest_bracket_search)
         furthest_bracket_search--;
     /* not on a bracket at all */
-    if (p == NULL)
+    if (p == NULL || *p == '\0')
         return -1;
     /* the matching bracket */
     d = p[1];