backend: Fix nil pointer when a process fails to start
backend: Fix nil pointer when a process fails to start

because the file cannot be found and process.Release must make
sure that field process is not nil.

corestoreio/caddy-esi,SchumacherFM/caddyesi,corestoreio/caddy-esi,SchumacherFM/caddyesi,SchumacherFM/caddyesi,corestoreio/caddy-esi