Wrapped Lua MapLoad hook in P_MapStart / P_MapEnd.

This fixes a crash if a script tries to use a MapLoad hook
to immediately edit the map in some way the moment it
finishes loading.

src/p_setup.c

@@ -2786,7 +2786,9 @@ boolean P_SetupLevel(boolean skipprecip)
 		}
 		P_PreTicker(2);
 #ifdef HAVE_BLUA
+		P_MapStart();
 		LUAh_MapLoad();
+		P_MapEnd();
 #endif
 	}