pacemaker: Express intention about logical or

Coverity shows warning about logical or on rv || EINVAL value. This
expression is not bug (and it is quite easy to read what is intended),
but may make sense to be more explicit using ternary
operator.

Also return EINVAL rather than -EINVAL, because pclose would return
positive errno value.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>