common: CephContext::_refresh_perf_values() checks for null _mempool_perf

common: CephContext::_refresh_perf_values() checks for null _mempool_perf

test_perf_counters_cache.cc initializes the CephContext with flag
CINIT_FLAG_NO_CCT_PERF_COUNTERS which leaves both _cct_perf and
_mempool_perf as nullptrs

the CephContextServiceThread calls _refresh_perf_values() regularly,
which guards _cct_perf->set() calls with a nullptr check, but the
_mempool_perf->set() calls were unguarded

this led to occasional unittest_perf_counters_cache segfaults in make
check

Fixes: https://tracker.ceph.com/issues/64895
Signed-off-by: Casey Bodley <cbodley@redhat.com>