FIX: Cause an error in PARSE for invalid args to TO

Previously, PARSE over string input would silently ignore invalid
arguments to TO. With this fix, invalid args to TO correctly cause an
error:
```
    >> parse "foo" [to 1.2]
    ** Script error: PARSE - invalid rule or usage of rule: to
    ** Where: parse
    ** Near: parse "foo" [to 1.2]
```
The problem was identified via static code analysis with Cppcheck.

(cherry picked from rebol#204)
fixes: Oldes/Rebol-issues#2129

src/core/u-parse.c

@@ -552,6 +552,8 @@ void Print_Parse_Index(REBCNT type, REBVAL *rules, REBSER *series, REBCNT index)
 				i = Find_Str_Bitset(series, 0, index, series->tail, 1, VAL_BITSET(item), HAS_CASE(parse));
 				if (i != NOT_FOUND && is_thru) i++;
 			}
+			else
+				Trap1(RE_PARSE_RULE, item - 1);
 		}
 	}
 

src/tests/units/parse-test.r3

@@ -19,6 +19,16 @@ Rebol [
 	--assert not parse "ba" [to a "ba"]
 	--assert parse/case "a" [thru a]
 	--assert not parse/case "A" [thru a]
+--test-- "TO/THRU with unsuported rule"
+;@@ https://github.com/Oldes/Rebol-issues/issues/2129
+	--assert all [
+		e: try [parse "foo" [thru some "0"]] ;@@ Red supports `some` right after `to`!
+		e/id = 'parse-rule
+	]
+	--assert all [
+		e: try [parse "foo" [thru 1.2]]
+		e/id = 'parse-rule
+	]
 
 
 ===end-group===