[crashlog] remove nullptr check on str_hw_reason

* given the earlier switch with a default to a numeric HW reboot reason,
  we are guaranteed that str_hw_reason is not null at this stage

Change-Id: Ifdccc6e257f7fadc6c52f8e9b0099e74bbc735a7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1530425
Commit-Queue: Francois Rousseau <frousseau@google.com>
Reviewed-by: John Grossman <johngro@google.com>
Fuchsia-Auto-Submit: Francois Rousseau <frousseau@google.com>