CHANGE: A plain, explicit QUIT from nested scripts returning UNSET! instead of 0

resolves: Oldes/Rebol-issues#2250

Author: Oldes

src/core/c-do.c

@@ -1835,7 +1835,8 @@ x*/	static REBINT Do_Args_Light(REBVAL *func, REBVAL *path, REBSER *block, REBCN
 
 	if (arg) {
 		if (IS_NONE(arg)) {
-			SET_INTEGER(TASK_THIS_VALUE, 0);
+			//SET_INTEGER(TASK_THIS_VALUE, 0); // for (do {quit}) == 0
+			SET_UNSET(TASK_THIS_VALUE);        // for (do {quit}) == unset!
 		} else 
 			*TASK_THIS_VALUE = *arg;	// save the value
 	} else {

src/tests/units/evaluation-test.r3

@@ -90,6 +90,11 @@ Rebol [
 		--assert unset? do %units/files/unset.r3
 		--assert dir = what-dir
 
+	--test-- "script with quit"
+	;@@ https://github.com/Oldes/Rebol-issues/issues/2250
+		--assert unset? do %units/files/quit.r3
+		--assert 42 = do %units/files/quit-return.r3
+
 ===end-group===
 
 ===start-group=== "do function"

src/tests/units/files/quit-return.r3

@@ -0,0 +1,3 @@
+Rebol [title: "Script which uses quit/return"] 
+quit/return 42
+print "Hello"

src/tests/units/files/quit.r3

@@ -0,0 +1,3 @@
+Rebol [title: "Script which uses quit"] 
+quit
+print "Hello"