Fix a crash triggered when firmware config takes too long

Fix a crash condition that is triggered when the firmware
configuration takes too long:
 1. The startup_timer expires, and it resets startup_future to NULL
 2. The delayed firmware_config_callback is received, and it
    tries to use the startup_future pointer (already set to NULL).

[Cherry-pick from AOSP]

Bug: 27336555
Change-Id: I4b04ca08a32c947f6f1eaabec7c4b099f96aab59