do not treat \0 as a backref in BRE

The valid BRE backref tokens are \1 .. \9, and 0 is not a special
character either so \0 is undefined by the standard.

Such undefined escaped characters are treated as literal characters
currently, following existing practice, so \0 is the same as 0.

src/regex/regcomp.c

@@ -839,7 +839,7 @@ static reg_errcode_t parse_atom(tre_parse_ctx_t *ctx, const char *s)
 			s--;
 			break;
 		default:
-			if (!ere && isdigit(*s)) {
+			if (!ere && (unsigned)*s-'1' < 9) {
 				/* back reference */
 				int val = *s - '0';
 				node = tre_ast_new_literal(ctx->mem, BACKREF, val, ctx->position);