DIRECTOR: LINGO: improve lingo c_not

It's a bit unexpected, but `not` works differently then the inverse of
`if`.

After testing in D4.0.4 the following two statements are true:

    defined variables are falsy when checked with not
        i.e. not varundefined => true and not vardefined => false
    only a var with value 0 is seen as false when checked with not
        i.e. not 0 => true

engines/director/lingo/lingo-code.cpp

@@ -1226,9 +1226,15 @@ void LC::c_or() {
 }
 
 void LC::c_not() {
+	// Not returns true when a variable is undefined or is an int and is zero.
+	Datum res;
 	Datum d = g_lingo->pop();
 
-	Datum res(d.asInt() == 0 ? 1 : 0);
+	if ((d.type == INT && d.u.i == 0) || d.type == VOID) {
+		res = Datum(1);
+	} else {
+		res = Datum(0);
+	}
 
 	g_lingo->push(res);
 }

engines/director/lingo/tests/if.lingo

@@ -7,6 +7,23 @@ if x = 6 then put 2
 
 set y = 4
 
+-- defined variables are falsy when checked with not
+-- i.e. not varundefined => true
+-- only a var with value 0 is seen as false when checked with not
+-- i.e. not 0 => true
+scummvmAssert(varundefined = 0)
+scummvmAssert(not varundefined = 1)
+
+set a to 0
+scummvmAssert(not a = 1)
+set a to 0.0
+scummvmAssert(not a = 0)
+set a to "string"
+scummvmAssert(not a = 0)
+set parray to [#pt: 1, #ar: 0]
+scummvmAssert(not parray = 0)
+--
+
 repeat with x = 1 to 6
 	if x = 3 then put 30
 	else if x = 4 then put 40