tarantool_instance doesn't do anything if tarantool_box isn't found

A fix for https://bugs.launchpad.net/tarantool/+bug/992171
"SAVE SNAPSHOT process (dumper) does not respond to signals"

Reset the signal mask/handler after a fork to their defaults.
Make sure that when the snapshot dumper process is terminated
with a signal, we report it back to the user.

No test case since it would be hard to do a concurrent
test in our framework yet.

When salloc_init() fails, we call tarantool_free(),
which calls fiber_free(), which tries to destroy the fiber registry
hash, which has not been allocated.

Check that the hash has been allocated, before trying to destroy it.