h264_mp4toannexb: fix the pps offset when there are more than one sps in avcc

the pps offset is used to locate pps in the spspps_buf; however, the
current calc method is wrong because it is the offset of the original
avctx->extradata;
when there is only one sps in the avcc; the value is correct by
coincidence, however, it will fail in avcc with multi sps
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_mp4toannexb_bsf.c

@@ -122,7 +122,7 @@ pps:
         if (!unit_nb && !sps_done++) {
             unit_nb = *extradata++; /* number of pps unit(s) */
             if (unit_nb) {
-                ctx->pps_offset = (extradata - 1) - (avctx->extradata + 4);
+                ctx->pps_offset = total_size;
                 pps_seen = 1;
             }
         }