Fix #68690: Hypothetical off-by-one condition

We fix this, even though `filter->cache == jisx0213_u2_tbl_len` can
never be true here.

Author: cmb69

ext/mbstring/libmbfl/filters/mbfilter_sjis_2004.c

@@ -508,7 +508,7 @@ mbfl_filt_conv_wchar_jis2004(int c, mbfl_convert_filter *filter) {
 
 	/* check for 2nd char of combining characters */
 	if ((filter->status & 0xf) == 1 &&
-		filter->cache >= 0 && filter->cache <= jisx0213_u2_tbl_len) {
+		filter->cache >= 0 && filter->cache < jisx0213_u2_tbl_len) {
 		k = filter->cache;
 		filter->status &= ~0xf;
 		filter->cache = 0;