The sender might provide all requested message bytes but no FD. The receiver
interface has no simple way of indicating this scenario yet, so just assert
out for now to ensure it never happens in the first place.
If needed, this can be changed to return a new error code to indicate partial
read in the future.
This allows using read_some for incoming messages with an optional FD.
Doing so fits the purpose of read_some more closely, which is to read *any*
non-empty amount of data.
FEXServer's logger performs such self-moves for the log pipe posix_descriptor
of short-lived clients. This resulted in a double-close previously, which
could interfere with file operations on other threads (typically crashing
FEXServer in effect).
LC