global/signal_handler: reset injected segv after test

global/signal_handler: reset injected segv after test

~CephContext() => ~TypedSingletonWrapper() => ~MempoolObs =>
unregister_command() => ldout() << "unregister_command" =>
Log::submit_entry() => *(volatile int *)(0) = 0xdead;

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit d932c8f2f23263924103a900714db82ee87f6eef)

Conflicts:
        src/log/Log.cc - jewel has "namespace log" instead of "namespace
                         logging" (trivial resolution)