Fix a "use before limits check" issue

This issue was reported by cppcheck.  We should be doing the limit check before
reading at the offset or we may at some point try to read off the end of the
array, potentially causing a segfault.

Author: Beirdo

mythtv/libs/libmythtv/cc708decoder.cpp

@@ -626,7 +626,7 @@ static void parse_cc_packet(CC708Reader* cb_cbs, CaptionPacket* pkt,
 
     assert(pkt_size<127);
 
-    while (pkt_buf[off] && off<pkt_size)
+    while (off < pkt_size && pkt_buf[off])
     { // service_block
         int block_size = pkt_buf[off] & 0x1f;
         service_number = (pkt_buf[off]>>5) & 0x7;