Install system exit callback only after initialization of shared memory.

Install system exit callback only after initialization of shared memory.

When the on-exit callback gets called because of a failure to acquire
shared memory, the cleanup function can produce a segfault while accessing
process_info, that lives in shared memory.
Although we can also fix this by bailing out from the exit callback when
process_info is NULL but installing the function after successful initialization
of shared memory is a better approach as the rest of the system always assumes
the process_info can never be NULL. Also, there is nothing to
clean up before child processes are spawned.