MINOR: quic: handle partially received buffered stream frame

MINOR: quic: handle partially received buffered stream frame

Adjust the function to handle buffered STREAM frames. If the offset of
the frame was already fully received, discard the frame. If only
partially received, compute the difference and copy only the newly
offset.

Before this change, a buffered frame representing a fully or partially
received offset caused the loop to be interrupted. The frame was
preserved, thus preventing frames with greater offset to be handled.

This may fix some occurences of stalled transfer on the request channel
if there is out-of-order STREAM frames on the Rx path.