avconv: Do not pass NULL to avio_tell

The null demuxer does not have a backing AVIOContext.

avconv.c

@@ -909,7 +909,7 @@ static void print_report(int is_last_report, int64_t timer_start)
     char buf[1024];
     OutputStream *ost;
     AVFormatContext *oc;
-    int64_t total_size;
+    int64_t total_size = 0;
     AVCodecContext *enc;
     int frame_number, vid, i;
     double bitrate, ti1, pts;
@@ -934,7 +934,7 @@ static void print_report(int is_last_report, int64_t timer_start)
 
 
     oc = output_files[0]->ctx;
-
+    if (oc->pb) {
         total_size = avio_size(oc->pb);
         if (total_size <= 0) // FIXME improve avio_size() so it works with non seekable output too
             total_size = avio_tell(oc->pb);
@@ -945,6 +945,7 @@ static void print_report(int is_last_report, int64_t timer_start)
                    "avio_tell() failed: %s\n", errbuf);
             total_size = 0;
         }
+    }
 
     buf[0] = '\0';
     ti1 = 1e10;