Don't crash if no asynchronous process has been created yet.

* src/process.c (wait_reading_process_output): Allow
child_signal_read_fd < 0.
5 jobs for master in 32 minutes and 12 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Prep Images
passed #10371
prep-image-base

00:01:34

 
  Build Images
passed #10372
build-image-inotify

00:20:37

 
  Fast
failed #10373
test-fast-inotify

00:10:00

 
  Normal
skipped #10374
test-lisp-inotify
skipped #10375
test-net-inotify
 
Name Stage Failure
failed
test-fast-inotify Fast
  src/xdisp-tests.log
Makefile:324: recipe for target 'check-doit' failed
make[1]: *** [check-doit] Error 1
make[1]: Leaving directory '/checkout/test'
Makefile:294: recipe for target 'check' failed
make: *** [check] Error 2
make: Leaving directory '/checkout/test'
Cleaning up file based variables
ERROR: Job failed: exit code 2