Made usage of assertions in VM code consistent

https://forum.zdoom.org/viewtopic.php?t=56995

src/dobject.h

@@ -198,7 +198,7 @@ protected:
 	enum { MetaClassNum = CLASSREG_PClass };
 
 	// Per-instance variables. There are four.
-#ifdef _DEBUG
+#ifndef NDEBUG
 public:
 	enum
 	{

src/scripting/vm/vm.h

@@ -376,7 +376,7 @@ int VMCall(VMFunction *func, VMValue *params, int numparams, VMReturn *results,
 // variable name <x> at position <p>
 void NullParam(const char *varname);
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 bool AssertObject(void * ob);
 #endif
 

src/scripting/vm/vmexec.cpp

@@ -229,7 +229,7 @@ void VMFillParams(VMValue *params, VMFrame *callee, int numparam)
 }
 
 
-#ifdef _DEBUG
+#ifndef NDEBUG
 bool AssertObject(void * ob)
 {
 	auto obj = (DObject*)ob;