Nails a fishing flaky to the wall (#88938)

## About this Pull Request
Closes #88833 
I have zero clue where the chrystarfish on the mount comes, or how it
got saved in UT's persistence SS in the first place (most likely source
of it), but this fixes the flaky.

Fishing is the only road to salvation this codebase has and it'll be our
downfall

code/modules/fishing/fish_mount.dm

@@ -52,6 +52,10 @@
 	else
 		RegisterSignal(SSfishing, COMSIG_SUBSYSTEM_POST_INITIALIZE, PROC_REF(load_trophy_fish))
 
+/obj/structure/fish_mount/Destroy(force)
+	QDEL_NULL(mounted_fish)
+	return ..()
+
 /obj/structure/fish_mount/proc/load_trophy_fish(datum/source)
 	SIGNAL_HANDLER
 	SSpersistence.load_trophy_fish(src)