rtmp: Don't blindly skip the 4 trailer bytes from the FLV packets

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>