Buffer overflow in the keymap UI

[AndreyB]

Fedora 25
Angband 4.1.0 (sdl front end)
Probably reproducible on most other platforms and front ends.

If you create a keymap exactly 20 characters long, the code triggers a read past the end of the array.
It seams the UB is lying dormant. I spotted it using -fsanitize=undefined. Better kill it now before it comes for us.

How to reproduce:
1. recompile with -fsanitize=undefined

$ make clean
$ SANITIZE_FLAGS="-fsanitize=undefined -fsanitize=address" ./configure
$ SANITIZE_FLAGS="-fsanitize=undefined -fsanitize=address" make

2. create a keymap
3. enter 20 characters (maximum allowed)
4. -fsanitize=undefined crashes the game, printing the diagnostic:

==1845==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000001090db0 at pc 0x000000696727 bp 0x7ffc2612be20 sp 0x7ffc2612be10
READ of size 4 at 0x000001090db0 thread T0
    #0 0x696726 in keypress_to_text /home/user/angband/src/ui-event.c:225
    #1 0x6e6b14 in ui_keymap_create /home/user/angband/src/ui-options.c:488
    #2 0x6c4a68 in menu_action_handle /home/user/angband/src/ui-menu.c:102
    #3 0x6cb9da in menu_handle_action /home/user/angband/src/ui-menu.c:661
    #4 0x6cd0a8 in menu_select /home/user/angband/src/ui-menu.c:797
    #5 0x6e7651 in do_cmd_keymaps /home/user/angband/src/ui-options.c:601
    #6 0x6c4a68 in menu_action_handle /home/user/angband/src/ui-menu.c:102
    #7 0x6cb9da in menu_handle_action /home/user/angband/src/ui-menu.c:661
    #8 0x6cd0a8 in menu_select /home/user/angband/src/ui-menu.c:797
    #9 0x6ee955 in do_cmd_options /home/user/angband/src/ui-options.c:1826
    #10 0x677a8c in do_cmd_xxx_options /home/user/angband/src/ui-command.c:138
    #11 0x697c89 in textui_process_command /home/user/angband/src/ui-game.c:313
    #12 0x697e02 in textui_get_cmd /home/user/angband/src/ui-game.c:325
    #13 0x698ccf in play_game /home/user/angband/src/ui-game.c:434
    #14 0x7679c4 in main /home/user/angband/src/main.c:524
    #15 0x7f841dba2400 in __libc_start_main (/lib64/libc.so.6+0x20400)
    #16 0x403769 in _start (/home/user/angband/src/angband+0x403769)

0x000001090db0 is located 0 bytes to the right of global variable 'keymap_buffer' defined in 'ui-options.c:377:24' (0x1090cc0) of size 240
0x000001090db0 is located 48 bytes to the left of global variable 'keymap_menu' defined in 'ui-options.c:574:21' (0x1090de0) of size 8
SUMMARY: AddressSanitizer: global-buffer-overflow /home/user/angband/src/ui-event.c:225 in keypress_to_text
Shadow bytes around the buggy address:
  0x00008020a160: 04 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9
  0x00008020a170: 04 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9
  0x00008020a180: 04 f9 f9 f9 f9 f9 f9 f9 01 f9 f9 f9 f9 f9 f9 f9
  0x00008020a190: 01 f9 f9 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
  0x00008020a1a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x00008020a1b0: 00 00 00 00 00 00[f9]f9 f9 f9 f9 f9 00 f9 f9 f9
  0x00008020a1c0: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
  0x00008020a1d0: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9 01 f9 f9 f9
  0x00008020a1e0: f9 f9 f9 f9 00 00 00 06 f9 f9 f9 f9 00 05 f9 f9
  0x00008020a1f0: f9 f9 f9 f9 00 02 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
  0x00008020a200: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==1845==ABORTING