FIX: crash when using do on a function which returns itself

resolves: Oldes/Rebol-issues#196
related to: Oldes/Rebol-issues#886 and Oldes/Rebol-issues#2440

Author: Oldes

src/core/c-do.c

@@ -932,6 +932,7 @@ x*/	static REBINT Do_Args_Light(REBVAL *func, REBVAL *path, REBSER *block, REBCN
 
 		// The return value is a FUNC that needs to be re-evaluated.
 		if (VAL_GET_OPT(DS_TOP, OPTS_REVAL) && ANY_FUNC(DS_TOP)) {
+			VAL_CLR_OPT(DS_TOP, OPTS_REVAL); // avoid recursion https://github.com/Oldes/Rebol-issues/issues/196
 			value = DS_POP; // WARNING: value is volatile on TOS1 !
 			word = Get_Type_Word(VAL_TYPE(value));
 			index--;		// Backup block index to re-evaluate.

src/tests/units/func-test.r3

@@ -258,9 +258,11 @@ Rebol [
 
 --test-- "issue-196"
 ;@@ https://github.com/Oldes/Rebol-issues/issues/196
-	; with change related to https://github.com/Oldes/Rebol-issues/issues/2440
-	; bellow test is now throwing error instead of `true`
-	--assert error? try [do func [a] [context? 'a] 1] ;-no crash
+;@@ https://github.com/Oldes/Rebol-issues/issues/886
+;@@ https://github.com/Oldes/Rebol-issues/issues/2440
+	--assert function? do func [/local a] [context? 'a] ;-no crash
+	--assert 1 = do has [arg] [1]
+	--assert function? do has [arg] [context? 'arg] ;-no crash and recursion
 
 --test-- "issue-216"
 ;@@ https://github.com/Oldes/Rebol-issues/issues/216