[JK2] Fix crash/hang when loading saved games on 64-bit builds

Add missing watchTarget pointer field to savefields_gNPC table.

The watchTarget field (gentity_t*) in gNPC_t was being serialized as
int32_t in sg_export() but was not included in the savefields_gNPC
conversion table. This caused:

- On save: 64-bit pointer truncated to 32-bit
- On load: Truncated value used as invalid pointer
- On use: Crash (Windows x64) or infinite loop (macOS ARM64) when
  NPC_BSCinematic() calls CalcEntitySpot(NPCInfo->watchTarget, ...)

The Jedi Academy codebase (code/game/g_savegame.cpp) correctly includes
watchTarget in its savefields_gNPC table, but the JK2 codebase was
missing this entry.

Fixes #1278

Author: dpiers

codeJK2/game/g_savegame.cpp

@@ -106,6 +106,7 @@ static const field_t savefields_gNPC[] =
 	{strNPCOFS(defendEnt),			F_GENTITY},
 	{strNPCOFS(greetEnt),			F_GENTITY},
 	{strNPCOFS(group),				F_GROUP},
+	{strNPCOFS(watchTarget),		F_GENTITY},
 
 	{NULL, 0, F_IGNORE}
 };
@@ -233,7 +234,7 @@ intptr_t GetGEntityNum(gentity_t* ent)
 
 gentity_t *GetGEntityPtr(intptr_t iEntNum)
 {
-	if (iEntNum == -1)
+	if (iEntNum < 0 || iEntNum >= MAX_GENTITIES)
 	{
 		return NULL;
 	}