h2645_parse: only read avc length code at the correct position

Reading it from any other position would result in a wrong size being
read, instead fallback to the re-sync mechanic in the else clause.

(cherry picked from commit c3e9b098)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h2645_parse.c

@@ -259,7 +259,7 @@ int ff_h2645_packet_split(H2645Packet *pkt, const uint8_t *buf, int length,
         int extract_length = 0;
         int skip_trailing_zeros = 1;
 
-        if (buf >= next_avc) {
+        if (buf == next_avc) {
             int i;
             for (i = 0; i < nal_length_size; i++)
                 extract_length = (extract_length << 8) | buf[i];
@@ -272,6 +272,9 @@ int ff_h2645_packet_split(H2645Packet *pkt, const uint8_t *buf, int length,
             }
             next_avc = buf + extract_length;
         } else {
+            if (buf > next_avc)
+                av_log(logctx, AV_LOG_WARNING, "Exceeded next NALFF position, re-syncing.\n");
+
             /* search start code */
             while (buf[0] != 0 || buf[1] != 0 || buf[2] != 1) {
                 ++buf;