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.