FIX: Negative /SKIP with FIND causing a hang

fixes: Oldes/Rebol-issues#2291

Author: Oldes

src/core/t-string.c

@@ -528,7 +528,10 @@ static REBSER *make_binary(REBVAL *arg, REBOOL make)
 
 		if (args & AM_FIND_PART) tail = index + Partial(value, 0, D_ARG(ARG_FIND_LENGTH), 0);
 		ret = 1; // skip size
-		if (args & AM_FIND_SKIP) ret = Partial(value, 0, D_ARG(ARG_FIND_SIZE), 0);
+		if (args & AM_FIND_SKIP) {
+			ret = Partial(value, 0, D_ARG(ARG_FIND_SIZE), 0);
+			if(!ret) goto is_none;
+		}
 
 		if (action == A_SELECT) args |= AM_FIND_TAIL;
 

src/tests/units/series-test.r3

@@ -78,6 +78,14 @@ Rebol [
 	--assert "b"  = find/case str #"b"
 	--assert none?  find/case str #"B"
 
+--test-- "FIND with negative skip"
+	;@@ https://github.com/Oldes/Rebol-issues/issues/2291
+	--assert none? find/skip "acdcde" "cd" -3
+	--assert "cde" = find/skip/reverse tail "acd000cde" "cd" -3
+	--assert "cde111" = find/skip/reverse tail "acd000cde111" "cd" -3
+	; I'm not sure with the next one, but I will keep it:
+	--assert "cde" = find/reverse/skip tail "acde" make bitset! #"c" -2
+
 --test-- https://github.com/Oldes/Rebol-issues/issues/66
 	--assert none? find/skip [1 2 3 4 5 6] 2 3