Check that length of data returned by sysctl is non-zero
The length of the data returned by sysctl can be zero, which was not checked for. This could cause crashes, e.g. when querying non-existent processes. (Bug#36279) * src/sysdep.c (list_system_processes) [DARWIN_OS || __FreeBSD__]: (system_process_attributes) [__FreeBSD__]: (system_process_attributes) [DARWIN_OS]: * src/filelock.c (get_boot_time) [CTL_KERN && KERN_BOOTTIME]: Check for zero length data returned by sysctl.