fix a race condition in UDPReader in QuicServerTest
fix a race condition in UDPReader in QuicServerTest

Summary:
if OnDataAvailable() gets called before the readOne(), then `bufPromise_` would
be null, and thus the available buffer (buf_) wouldn't get appended to the
bufPromise_.

Reviewed By: sharma95

Differential Revision: D15960463

fbshipit-source-id: b48df104a021a9e89e863d09a0124c6b19228113

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