media: av7110: prevent underflow in write_ts_to_decoder()

In the Linux kernel, the following vulnerability has been resolved: media: av7110: prevent underflow in write_ts_to_decoder() The buf[4] value comes from the user via ts_play(). It is a value in the u8 range. The final length we pass to av7110_ipack_instant_repack() is "len - (buf[4] + 1) - 4" so add a check to ensure that the length is not negative. It's not clear that passing a negative len value does anything bad necessarily, but it's not best practice. With the new bounds checking the "if (!len)" condition is no longer possible or required so remove that.