SF #11 Caption is not checking for bounds of class and c2 correctly

This fixes some array bounds checks that were off by one in the
xds_separator function in caption.c. If class == 4 or c2 == 24 then the
xds_sub_packet "sp" would be set from memory outside the cc->sub_packet
array.

Closes Github issue #16

src/caption.c

@@ -610,8 +610,8 @@ xds_separator(vbi_decoder *vbi, uint8_t *buf)
  case 1 ... 14:
  class = (c1 - 1) >> 1;
 
- if (class > elements(cc->sub_packet)
- || c2 > (int) elements(cc->sub_packet[0])) {
+ if (class >= elements(cc->sub_packet)
+ || c2 >= (int) elements(cc->sub_packet[0])) {
  XDS_SEP_DEBUG(printf("XDS ignore packet %d/0x%02x\n", class, c2));
  cc->curr_sp = NULL;
  return;