PM: EM: Fix NULL pointer dereference when perf domain ID is not found

In the Linux kernel, the following vulnerability has been resolved:

PM: EM: Fix NULL pointer dereference when perf domain ID is not found

dev_energymodel_nl_get_perf_domains_doit() calls
em_perf_domain_get_by_id() but does not check the return value before
passing it to __em_nl_get_pd_size(). When a caller supplies a
non-existent perf domain ID, em_perf_domain_get_by_id() returns NULL,
and __em_nl_get_pd_size() immediately dereferences pd->cpus
(struct offset 0x30), causing a NULL pointer dereference.

The sister handler dev_energymodel_nl_get_perf_table_doit() already
handles this correctly via __em_nl_get_pd_table_id(), which returns
NULL and causes the caller to return -EINVAL. Add the same NULL check
in the get-perf-domains do handler.

[ rjw: Subject and changelog edits ]