Don't freeze with unreadable processes

Don't freeze with unreadable processes

Don't freeze if an exiting process can't be read from. (Bug#19860).
This fixes a bug I introduced in
2014-07-08T07:24:07Z@eggert@cs.ucla.edu
"* process.c: Add sanity checks for file descriptors."
Dmitry Gutov did most of the legwork in finding the problem.
* src/process.c (wait_reading_process_output):
Treat non-running processes that can't be read from
the same as other non-running processes.