game.cpp: Fix loading commander droids without psGroup

When a commander transitions from an offworld mission,
it will currently lose its group (actually, this is kind of
a bug, too), i.e. its `psGroup` is `nullptr`.

If a player attempts to save the game right after
this transition and then loads the savegame again,
the game will crash.

`loadSaveDroid()` function will try to allocate
a new group for these "orphaned" returning
commanders.

But this new group ID is not in sync with other
group IDs that may already be present in the
currently-processed "droid.json" file and thus
may clash with other groups, leading to various
memory-related bugs.

For example, this may happen when there is
a commander back at the players' base with
some commander group assigned.

There is a non-zero probability that the
commander which returns from an offworld
mission, will try to allocate the same group ID
as the commander back at the base.

Signed-off-by: Pavel Solodovnikov <[email protected]>

src/game.cpp

@@ -5552,10 +5552,16 @@ static bool loadSaveDroid(const char *pFileName, PerPlayerDroidLists& ppsCurrent
 	// Sort list so transports are loaded first, since they must be loaded before the droids they contain.
 	std::vector<std::pair<int, WzString>> sortedList;
 	bool missionList = fName.compare("mdroid");
+	// This will be set to the largest group ID found throughout the droid file + 1.
+	//
+	// This helps to allocate non-conflicting group IDs for commanders which
+	// happen to lose their group, e.g. when transitioning from an offworld mission.
+	int nextFreeGroupID = 0;
 	for (size_t i = 0; i < list.size(); ++i)
 	{
 		ini.beginGroup(list[i]);
 		DROID_TYPE droidType = (DROID_TYPE)ini.value("droidType").toInt();
+		int aigroup = ini.value("aigroup", -1).toInt();
 		int priority = 0;
 		switch (droidType)
 		{
@@ -5571,6 +5577,10 @@ static bool loadSaveDroid(const char *pFileName, PerPlayerDroidLists& ppsCurrent
 			{
 				++priority;
 			}
+			if (aigroup >= 0)
+			{
+				nextFreeGroupID = std::max(nextFreeGroupID, aigroup) + 1;
+			}
 		default:
 			break;
 		}
@@ -5701,7 +5711,7 @@ static bool loadSaveDroid(const char *pFileName, PerPlayerDroidLists& ppsCurrent
 		{
 			if (isTransporter(psDroid) || psDroid->droidType == DROID_COMMAND)
 			{
-				DROID_GROUP *psGroup = grpCreate();
+				DROID_GROUP *psGroup = grpCreate(nextFreeGroupID++);
 				psGroup->add(psDroid);
 			}
 			else