The qSpeedTest packet is used for experiments to determine the optimal
packet size for a given communication medium, e.g. to transfer 10MB of
memory, is it faster to send a hundred 100KB packets or ten 1MB packets.
It creates a packet of the requested size in a stack allocation, but is
not checking that its buffer is large enough for the requested size.
Change this allocation to be on heap, and impose a maximum size that can
be tested (4MB, for now).
rdar://158630250
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