main/config_options: Check for existance of internal object before derefing

main/config_options: Check for existance of internal object before derefing

Asterisk can load and register an object type while still having an invalid
sorcery mapping. This can cause an issue when a creation call is invoked.
For example, mis-configuring PJSIP's endpoint identifier by IP address mapping
in sorcery.conf will cause the sorcery mechanism to be invalidated; however, a
subsequent ARI invocation to create the object will cause a crash, as the
internal type may not be registered as sorcery expects.

Merely checking for a NULL pointer here solves the issue.

Change-Id: I54079fb94a1440992f4735a9a1bbf1abb1c601ac