S_scan_ident: Add an assertion

That this had to be true was not obvious to me without studying closely
the code before it.  Adding an assertion will result in others deciding
they don't have to figure it out.

Author: khwilliamson

toke.c

@@ -10878,6 +10878,7 @@ S_scan_ident(pTHX_ char *s, char *dest, char *dest_end, U32 flags)
      * If that copied character is a digit, it means we have something like
      * ${10}, ${1547}, etc.  Handle those the same way we handle $1, etc */
     if (isDIGIT(*d)) {
+        assert(bracket != NO_BRACE);
         s = parse_ident(s - 1, PL_bufend, &d, e, is_utf8,
                         STOP_AT_FIRST_NON_DIGIT);