Check type of Squirrel constructor_stub() instance

This prevents manual invocations of the native class constructor with an
invalid value.

Author: z33ky

sp/src/vscript/vscript_squirrel.cpp

@@ -1569,15 +1569,23 @@ SQInteger constructor_stub(HSQUIRRELVM vm)
 	Assert(pSquirrelVM);
 
 	sq_resetobject(&pSquirrelVM->lastError_);
+	{
+		SQUserPointer p;
+		if (SQ_FAILED(sq_getinstanceup(vm, 1, &p, 0)))
+		{
+			return sq_throwerror(vm, "Expected class userpointer");
+		}
 
-	void* instance = pClassDesc->m_pfnConstruct();
+		if (!p)
+		{
+			return sq_throwerror(vm, "Accessed null instance");
+		}
 
-	// expect construction to always succeed
-	Assert(sq_isnull(pSquirrelVM->lastError_));
+		void* instance = pClassDesc->m_pfnConstruct();
+
+		// expect construction to always succeed
+		Assert(sq_isnull(pSquirrelVM->lastError_));
 
-	{
-		SQUserPointer p;
-		sq_getinstanceup(vm, 1, &p, 0);
 		new(p) ClassInstanceData(instance, pClassDesc, nullptr, true);
 	}