avformat/vivo: Don't log null value

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/vivo.c

@@ -166,7 +166,7 @@ static int vivo_read_header(AVFormatContext *s)
             value = strchr(key, ':');
             if (!value) {
                 av_log(s, AV_LOG_WARNING, "missing colon in key:value pair '%s'\n",
-                       value);
+                       key);
                 continue;
             }