SoundFile.cpp: do a NULL check before deferencing.

CID 296.

src/kits/media/SoundFile.cpp

@@ -360,6 +360,10 @@ BSoundFile::_ref_to_file(const entry_ref *ref)
 	if (mf.type == B_MEDIA_RAW_AUDIO) {
 		raw = &mf.u.raw_audio;
 	}
+
+	if (raw == NULL)
+		return B_ERROR;
+
 	fSamplingRate = (int)raw->frame_rate;
 	fChannelCount = raw->channel_count;
 	fSampleSize = raw->format & 0xf;