movespeed_modification unlawful list modification fix (#50135)

* copy

* Spooky suggestion

code/__HELPERS/_lists.dm

@@ -11,6 +11,7 @@
 
 #define LAZYINITLIST(L) if (!L) L = list()
 #define UNSETEMPTY(L) if (L && !length(L)) L = null
+#define LAZYCOPY(L) (L ? L.Copy() : list() )
 #define LAZYREMOVE(L, I) if(L) { L -= I; if(!length(L)) { L = null; } }
 #define LAZYADD(L, I) if(!L) { L = list(); } L += I;
 #define LAZYOR(L, I) if(!L) { L = list(); } L |= I;

code/modules/movespeed/_movespeed_modifier.dm

@@ -199,7 +199,7 @@ GLOBAL_LIST_EMPTY(movespeed_modification_cache)
 
 /// Get the move speed modifiers list of the mob
 /mob/proc/get_movespeed_modifiers()
-	. = movespeed_modification
+	. = LAZYCOPY(movespeed_modification)
 	for(var/id in movespeed_mod_immunities)
 		. -= id