processes exits at the same time. The linux_emuldata structure is freed
but p->p_emuldata is left as a dangling pointer to the just freed memory.
The check for W_EXIT in the loop scanning the child processes isn't safe
since the state of the child process can change right afterwards. Lock
the process and check the W_EXIT before delivering signal.
Submitted by: tegge
Reviewed by: davidxu
MFC after: 1 week
17b9edd35a