Fixed excessive growth of ACS string pool

This fixes usage of uninitialized variable in ACSStringPool::PoolEntry objects
The initial version (before 66d15dc) increased pool size by one entry and assign all its members right after that
The improved version reserved MIN_GC_SIZE entries but didn't initialize anything except the first one
ACSStringPool::FindFirstFreeEntry() cannot find the proper entry as it uses PoolEntry::Next member for list traversal
It's enough to initialize Next member with FREE_ENTRY value because other fields will be assigned anyway inside ACSStringPool::InsertString()

https://forum.zdoom.org/viewtopic.php?t=60049

src/p_acs.h

@@ -129,7 +129,7 @@ private:
 	{
 		FString Str;
 		unsigned int Hash;
-		unsigned int Next;
+		unsigned int Next = FREE_ENTRY;
 		bool Mark;
 		TArray<int> Locks;