If not enough bytes are available, keep track of them and skip
them on next call.

In practice, if these trailer bytes are written in a separate
call, there is no other data written in this call, making it
fall into the "FLV packet too small" case currently - working,
but not as intended.

This patch makes the code more robust, handling all cases
except for having the FLV packet header split over multiple
write calls.

Signed-off-by: Martin Storsjö <martin@martin.st>