avformat/rtpdec_mpa_robust: change assignment to inequality test in conditional

In the spirit of commit 8199908f, likely
typo originally.

Found by enabling -Wparentheses on clang 3.7 and running a manual audit.
To the best of my knowledge, no such instances remain.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavformat/rtpdec_mpa_robust.c

@@ -166,7 +166,7 @@ static int mpa_robust_parse_packet(AVFormatContext *ctx, PayloadContext *data,
             "Received packet without a start fragment; dropping.\n");
         return AVERROR(EAGAIN);
     }
-    if (adu_size = data->adu_size ||
+    if (adu_size != data->adu_size ||
         data->timestamp != *timestamp) {
         ffio_free_dyn_buf(&data->fragment);
         av_log(ctx, AV_LOG_ERROR, "Invalid packet received\n");