inputflinger: Check for KeyCharacterMap before use

Fixes crash in InputReader:

10-09 23:27:30.342 F DEBUG   : pid: 2243, tid: 2413, name: InputReader  >>> system_server <<<
10-09 23:27:30.342 F DEBUG   : uid: 1000
10-09 23:27:30.342 F DEBUG   : tagged_addr_ctrl: 0000000000000001 (PR_TAGGED_ADDR_ENABLE)
10-09 23:27:30.342 F DEBUG   : signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0000000000000040
10-09 23:27:30.342 F DEBUG   : Cause: null pointer dereference

Bug: 372558955
Test: Does not crash on input device without character map
Flag: EXEMPT bugfix
Change-Id: I0f4ada4663bdd51a233fa9c15eb2558db0081dd5