avformat/wtvdec: Forward errors when reading packet

wtvfile_read_packet did not abide by the requirements of an
AVIOContext's read_packet-function: If it did not read anything,
it returned zero, which currently leads to a warning in read_packet_wrapper
in aviobuf.c. Said warning will be an av_assert2 as soon as
FF_API_OLD_AVIO_EOF_0 is zero (probably the next major version bump).
So instead forward the error code from the underlying protocol.

This error/assert is triggered in the wtv-demux FATE test.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/wtvdec.c

@@ -71,7 +71,7 @@ static int wtvfile_read_packet(void *opaque, uint8_t *buf, int buf_size)
 {
     WtvFile *wf = opaque;
     AVIOContext *pb = wf->pb_filesystem;
-    int nread = 0;
+    int nread = 0, n = 0;
 
     if (wf->error || pb->error)
         return -1;
@@ -80,7 +80,6 @@ static int wtvfile_read_packet(void *opaque, uint8_t *buf, int buf_size)
 
     buf_size = FFMIN(buf_size, wf->length - wf->position);
     while(nread < buf_size) {
-        int n;
         int remaining_in_sector = (1 << wf->sector_bits) - (wf->position & ((1 << wf->sector_bits) - 1));
         int read_request        = FFMIN(buf_size - nread, remaining_in_sector);
 
@@ -100,7 +99,7 @@ static int wtvfile_read_packet(void *opaque, uint8_t *buf, int buf_size)
             }
         }
     }
-    return nread;
+    return nread ? nread : n;
 }
 
 /**