fix SIGABRT from NANs and json in bm_estimate_time
fix SIGABRT from NANs and json in bm_estimate_time

Summary:
Use first trial results if bm_estimate_time's p25 is the same as it's p75 instead of returning empty counters and NAN.

This should avoid a NAN being propagated up to json pretty print and causing this error in the event json output and bm_estimate_time are used together.

Reviewed By: dmm-fb

Differential Revision: D40027691

fbshipit-source-id: 028472e2be4e06b8c46a0c7fd6e88aba454762c8

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