fix: quickfix for bash-related crashes

[chaosvolt]

Purpose of change

Fix a case of nullpointer fuckery causing random crashes. Fixes #4236

Describe the solution

In map.cpp, set map::smash_items so the part of the check that looks for zombify_into is properly nested behind an is_corpse check, to avoid the risk that get_mtype will implode over something that won't have it.

Describe alternatives you've considered

Screaming.

Testing

1. Compiled and load-tested.
2. Spawned in a bunch of steel targets and shot the fuck out of them with a barret, no crashes so far.
3. Spawned in a road roller and rammed it at highway speed into a lone of hay bales, no crashes.
4. Ran over some NPCs, ran over generated human corpses, and shot a target spot with a corpse on it with both a barret and an mk19, no crashes so far.
5. Dropped C-4 and a mininuke on a NPCs and their bodies, also no crashes.
6. Checked affected file for astyle.

Additional context

Lemme know if I missed any other obvious fuckups.

Checklist