Fixed segfault caused by attitude check when monster shooting vehicle

[Vollch]

Summary

SUMMARY: Bugfixes "Fixed segfault caused by attitude check when monster shooting vehicle"

Purpose of change

When monster target_moving_vehicles it legitimely have nullptr target, and those untargeted shoots cause segfault due to friendliness check.

Describe the solution

Added null check.

Describe alternatives you've considered

Pointing author at issue so i don't need to fill exhausting surveys.

Testing

Loaded bugged save, no more crashes.

Additional context

Thread 1 received signal SIGSEGV, Segmentation fault.
0x00007ff611faf9fc in monster::attitude_to (this=0x31d32840, other=...) at src/monster.cpp:1047
1047        const monster *m = other.is_monster() ? static_cast< const monster *>( &other ) : nullptr;
(gdb) p other
$1 = (const Creature &) <error reading variable: Cannot access memory at address 0x0>
(gdb) bt
#0  0x00007ff611faf9fc in monster::attitude_to (this=0x31d32840, other=...) at src/monster.cpp:1047
#1  0x00007ff611e9a0e0 in gun_actor::call (this=0x18d912e0, z=...) at src/mattack_actors.cpp:585
#2  0x00007ff611f976c7 in monster::move (this=0x31d32840) at src/monmove.cpp:751
#3  0x00007ff611a01f8b in game::monmove (this=0x914bd10) at src/game.cpp:4125
#4  0x00007ff6119eb6d9 in game::do_turn (this=0x914bd10) at src/game.cpp:1505
#5  0x00007ff611d87951 in main (argc=0, argv=0xb1fb8) at src/main.cpp:764
(gdb) f 1
#1  0x00007ff611e9a0e0 in gun_actor::call (this=0x18d912e0, z=...) at src/mattack_actors.cpp:585
585         if( z.attitude_to( *target ) == Creature::A_FRIENDLY ) {
(gdb) p untargeted
$2 = true
(gdb) p target_moving_vehicles
$3 = true