cpufreq: prevent NULL dereference in cpufreq_online()

Ensure cpufreq_driver->set_boost is non-NULL before using it in
cpufreq_online() to prevent a potential NULL pointer dereference.

Reported-by: Gautam Menghani <gautam@linux.ibm.com>
Closes: https://lore.kernel.org/all/c9e56c5f54cc33338762c94e9bed7b5a0d5de812.camel@linux.ibm.com/


Fixes: dd016f37 ("cpufreq: Introduce a more generic way to set default per-policy boost flag")
Suggested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Link: https://patch.msgid.link/20250205181347.2079272-1-aboorvad@linux.ibm.com


[ rjw: Minor edits in the subject and changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>