avformat/rtsp: do not log invalid values

When reading fails the first time, ch would be uninitialized and
printed in the log message. Instead check for an error early and
log it properly.

libavformat/rtsp.c

@@ -1239,9 +1239,12 @@ start:
         q = buf;
         for (;;) {
             ret = ffurl_read_complete(rt->rtsp_hd, &ch, 1);
+            if (ret != 1) {
+                ret = (ret < 0) ? ret : AVERROR(EIO);
+                av_log(s, AV_LOG_WARNING, "Failed reading RTSP data: %s\n", av_err2str(ret));
+                return ret;
+            }
             av_log(s, AV_LOG_TRACE, "ret=%d c=%02x [%c]\n", ret, ch, ch);
-            if (ret != 1)
-                return ret < 0 ? ret : AVERROR(EIO);
             if (ch == '\n')
                 break;
             if (ch == '$' && q == buf) {