lavf/libssh: translate a read of 0 to EOF

Yet another case of forgotten 0 =! EOF translation.

While the documentation for this specific synchronous read
function does not mention it, the documentation for
`sftp_async_read` documents it, as well as looking at the
implementation of this function leads one to find
`if (handle->eof) { return 0; }`.

Reported by stnutt on IRC.

(cherry picked from commit 26892c7615)

libavformat/libssh.c

@@ -295,7 +295,7 @@ static int libssh_read(URLContext *h, unsigned char *buf, int size)
         av_log(libssh, AV_LOG_ERROR, "Read error.\n");
         return AVERROR(EIO);
     }
-    return bytes_read;
+    return bytes_read ? bytes_read : AVERROR_EOF;
 }
 
 static int libssh_write(URLContext *h, const unsigned char *buf, int size)