Merc arrival location fix (#263)

* User center insertion point for merc arrival

instead of initial arrival gridno that is meant only for the very first helidrop arrival at the start of a game.

* Prevent possible nullptr dereference

Ja2/gamescreen.cpp

@@ -628,7 +628,14 @@ UINT32	MainGameScreenHandle(void)
 				InternalLocateGridNo( gGameUBOptions.LOCATEGRIDNO, TRUE );
         }
 #else
+		if (gfFirstHeliRun)
+		{
 			InternalLocateGridNo( gGameExternalOptions.iInitialMercArrivalLocation, TRUE );
+		}
+		else
+		{
+			InternalLocateGridNo(gMapInformation.sCenterGridNo, TRUE);
+		}
 #endif
 		// Flugente: we might have reloaded the game, so we are currently not dropping mercs out of a helicopter
 		gfIngagedInDrop = FALSE;