fixed a bug in the original engine causing memory-protected systems to occasionally crash

svn-id: r25614

1 files changed, 13 insertions, 5 deletions

diff --git a/engines/parallaction/commands.cpp b/engines/parallaction/commands.cpp
index fada14c94e..44910b35aa 100644
--- a/engines/parallaction/commands.cpp
+++ b/engines/parallaction/commands.cpp
@@ -295,11 +295,19 @@ void runCommands(Command *list, Zone *z) {
 			break;
 
 		case CMD_ON:	// on
-			u->_zone->_flags &= ~kFlagsRemove;
-			u->_zone->_flags |= kFlagsActive;
-			if ((u->_zone->_type & 0xFFFF) == kZoneGet) {
-				addJob(&jobDisplayDroppedItem, u->_zone, kPriority17 );
-			}
+            // WORKAROUND: the original DOS-based engine didn't check u->_zone before dereferencing
+            // the pointer to get structure members, thus leading to crashes in systems with memory
+            // protection.
+            // As a side note, the overwritten address is the 5th entry in the DOS interrupt table
+            // (print screen handler): this suggests that a system would hang when the print screen
+            // key is pressed after playing Nippon Safes, provided that this code path is taken.
+            if (u->_zone != NULL) {
+                u->_zone->_flags &= ~kFlagsRemove;
+                u->_zone->_flags |= kFlagsActive;
+                if ((u->_zone->_type & 0xFFFF) == kZoneGet) {
+                    addJob(&jobDisplayDroppedItem, u->_zone, kPriority17 );
+                }
+            }
 			break;
 
 		case CMD_OFF:	// off