fix nil ptr

[dvovk]

This pull request includes a small but important change to the perCPUTimes function in the cpu/cpu_darwin.go file. The change adds a check to ensure that the cpuload variable is not nil before proceeding, which helps prevent potential runtime errors.

• cpu/cpu_darwin.go: Added a nil check for the cpuload variable in the perCPUTimes function to handle cases where host_processor_info might return a nil value.

Possible fix for: #1834

[shirou]

Good catch. Thank you so much!