Handle END or past-end cases for ?? in PARSE

The ?? debugging command in PARSE shows the current rule and
the current position in the data.  Both of these could be at the end of
series, which are not currently legal to pass to the molding of values.
This adds very basic handling for the end conditions in custom print
messages for those cases.

src/core/u-parse.c

@@ -64,12 +64,34 @@ enum parse_flags {
 
 static REBCNT Parse_Rules_Loop(REBPARSE *parse, REBCNT index, const REBVAL *rules, REBCNT depth);
 
-void Print_Parse_Index(enum Reb_Kind type, const REBVAL *rules, REBSER *series, REBCNT index)
-{
-    REBVAL val;
-    Val_Init_Series(&val, type, series);
-    VAL_INDEX(&val) = index;
-    Debug_Fmt("%r: %r", rules, &val);
+void Print_Parse_Index(
+    enum Reb_Kind type,
+    const REBVAL rule[], // positioned at the current rule
+    REBSER *series,
+    REBCNT index
+) {
+    REBVAL item;
+    Val_Init_Series(&item, type, series);
+    VAL_INDEX(&item) = index;
+
+    // Either the rules or the data could be positioned at the end.  The
+    // data might even be past the end.
+    //
+    // !!! Or does PARSE adjust to ensure it never is past the end, e.g.
+    // when seeking a position given in a variable or modifying?
+    //
+    if (IS_END(rule)) {
+        if (index >= SERIES_LEN(series))
+            Debug_Fmt("[]: ** END **");
+        else
+            Debug_Fmt("[]: %r", &item);
+    }
+    else {
+        if (index >= SERIES_LEN(series))
+            Debug_Fmt("%r: ** END **", rule);
+        else
+            Debug_Fmt("%r: %r", rule, &item);
+    }
 }