Commit 9f5a583694396f84 ("spawn (and thus popen_rd) die on failure")
was incomplete in that it only removed error checking for spawn
failures for non-(vfork|fork) calls, but the actual (vfork|fork)
PID result could still be undef.
Fixes: 9f5a583694396f84 ("spawn (and thus popen_rd) die on failure")