Save the current desegmentation state and restore it instead of
setting it back to zero. This is needed in the case of RFC 4751
RTP framed over TCP, where multiple RTP packets are contained in
one TCP/IP frame, and the preference to check for RTP payload
segmentation is enabled as well (as it is by default.)

That's somewhat unusual, but it could happen if RTP packets over
UDP use fragmentation, and then are individually encapsulated in
TCP (without combining the payload) for forwarding by a middlebox,
and then something like TSO is used.

Fix #19345