Memory errors when loading map that changes weather

I discovered this bug because people were unable to join my server when it was on Stormy Castle Zone.

I discovered that this is related to precipation by looking at the output of valgrind

I created a minimal reproducer: test.wad

Steps to reproduce:

1. srb2 -file test.wad -server
2. wait for the map to load
3. srb2 -connect localhost
4. Observe that the second srb2 instance crashes with a segfault or invalid backwards link.

Removing the change weather action on the linedef resolves this issue.