xkb: Always use MAP_LENGTH keymap size

Generating the modifier modmap, the helper function generate_modkeymap() would check the entire range up to the MAP_LENGTH.

However, the given keymap might have less keycodes than MAP_LENGTH, in which case we would go beyond the size of the modmap, as reported by ASAN:

==ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 1 at 0x5110001c225b thread T0
    #0 0x5e7369393873 in generate_modkeymap ../dix/inpututils.c:309
    #1 0x5e736930dcce in ProcGetModifierMapping ../dix/devices.c:1794
    #2 0x5e7369336489 in Dispatch ../dix/dispatch.c:550
    #3 0x5e736934407d in dix_main ../dix/main.c:275
    #5 0x7e46d47b2ecb in __libc_start_main
    #6 0x5e73691be324 in _start (xserver/build/hw/xwayland/Xwayland)

Address is located 0 bytes after 219-byte region
allocated by thread T0 here:
    #0 0x7e46d4cfc542 in realloc
    #1 0x5e73695aa90e in _XkbCopyClientMap ../xkb/xkbUtils.c:1142
    #2 0x5e73695aa90e in XkbCopyKeymap ../xkb/xkbUtils.c:1966
    #3 0x5e73695b1b2f in XkbDeviceApplyKeymap ../xkb/xkbUtils.c:2023
    #4 0x5e73691c6c18 in keyboard_handle_keymap ../hw/xwayland/xwayland-input.c:1194

As MAP_LENGTH is used in various code paths where the max keycode might not be easily available, best is to always use MAP_LENGTH to allocate the keymaps so that the code never run past the buffer size.

If the max key code is smaller than the MAP_LENGTH limit, fill-in the gap with zeros.

That also simplifies the code slightly as we do not constantly need to reallocate the keymap to adjust to the max key code size.

Closes: #1780 (closed)
Signed-off-by: Olivier Fourdan ofourdan@redhat.com