No functional change expected.
The error was only non-nil if some required values for the STOP_WAITING
frame were not set. It should be sufficient to throw an error when
attempting to write an invalid STOP_WAITING frame.
We should check if the rest of the STREAM frame contains enough bytes to
read the full data length, not if this overflows the MaxPacketSize
(which is the maximum packet size we use for sending, and has nothing to
do with receiving packets).
