sftp: fix read

libssh2_sftp_read may return less than requested.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 56d7e5528a8c35bd7d515049e90117d2ff262fcc)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/access/sftp.c

@@ -299,6 +299,7 @@ static block_t* Block( access_t* p_access )
     }
     else
     {
+        p_block->i_buffer = i_ret;
         p_access->info.i_pos += i_ret;
         return p_block;
     }