Avoid division by zero in circuitstats pareto

In circuit_build_times_calculate_timeout() in circuitstats.c, avoid dividing
by zero in the pareto calculations.

If either the alpha or p parameters are 0, we would divide by zero, yielding
an infinite result; which would be clamped to INT32_MAX anyway. So rather
than dividing by zero, we just skip the offending calculation(s), and
use INT32_MAX for the result.

Division by zero traps under clang -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error.
Stop spawn test failures due to a race condition with SIGCHLD on process exit

When a spawned process forks, fails, then exits very quickly, (this
typically occurs when exec fails), there is a race condition between the
SIGCHLD handler updating the process_handle's fields, and checking the
process status in those fields. The update can occur before or after the
spawn tests check the process status.

We check whether the process is running or not running (rather than just
checking if it is running) to avoid this issue.
Merge branch 'test-spawn-fail-race-condition'
Merge branch 'circuitstats-pareto-avoid-div-zero'