rtpdec: Don't pass a non-AVClass pointer as log context

The log context is assumed to start with an AVClass pointer.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec.c

@@ -785,7 +785,7 @@ int ff_parse_fmtp(AVStream *stream, PayloadContext *data, const char *p,
     int value_size = strlen(p) + 1;
 
     if (!(value = av_malloc(value_size))) {
-        av_log(stream, AV_LOG_ERROR, "Failed to allocate data for FMTP.");
+        av_log(NULL, AV_LOG_ERROR, "Failed to allocate data for FMTP.");
         return AVERROR(ENOMEM);
     }