Fixes immobilized mobs being able to buckle onto chairs (#5317)

# About the pull request

#5277, fixes the issue outlined here, incapacitated marines should not
be able to buckle themselves.

# Explain why it's good for the game

bug bad

# Changelog

:cl:
fix: fixes immobilized mobs being able to buckle themselves
/:cl:

code/game/objects/objs.dm

@@ -278,6 +278,11 @@
 	if (iszombie(user))
 		return
 
+	// mobs that become immobilized should not be able to buckle themselves.
+	if(M == user && HAS_TRAIT(user, TRAIT_IMMOBILIZED))
+		to_chat(user, SPAN_WARNING("You are unable to do this in your current state."))
+		return
+
 	if(density)
 		density = FALSE
 		if(!step(M, get_dir(M, src)) && loc != M.loc)