go/libraries/doltcore/remotestorage: internal/reliable: Recv: Fix a race where a completed state machine run and a canceled parent context could return a nil response message with a nil error.

[coffeegoddd]

@reltuk DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
f6ee0ff	ok	5937457

version	total_tests
f6ee0ff	5937457

correctness_percentage
100.0