Fix any possible division by zero errors

[brucecrevensten]

From Cloudwatch:

[ERROR] ZeroDivisionError: 0.0 cannot be raised to a negative power
Traceback (most recent call last):
  File "/var/task/lambda_function.py", line 586, in lambda_handler
    realtime_threshold_upper = landslide_threshold(realtime_antecedent)
  File "/var/task/lambda_function.py", line 64, in landslide_threshold
    return m * antecedent_mm**b

Check any other division operators for similar conditions. What in the upstream data caused this one?