Make AddItem's quantity overflow on negative numbers

CHANGELOG.md

@@ -115,6 +115,7 @@
     Bug #5906: Sunglare doesn't work with Mesa drivers and AMD GPUs
     Bug #5912: ImprovedBound mod doesn't work
     Bug #5914: BM: The Swimmer can't reach destination
+    Bug #5934: AddItem command doesn't accept negative values
     Feature #390: 3rd person look "over the shoulder"
     Feature #832: OpenMW-CS: Handle deleted references
     Feature #1536: Show more information about level on menu

apps/openmw/mwscript/containerextensions.cpp

@@ -93,7 +93,7 @@ namespace MWScript
                     runtime.pop();
 
                     if (count<0)
-                        throw std::runtime_error ("second argument for AddItem must be non-negative");
+                        count = static_cast<uint16_t>(count);
 
                     // no-op
                     if (count == 0)