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.
Merge remote-tracking branch 'teor/circuitstats-pareto-avoid-div-zero'
Note when 13290 was introduced.