net: lib: lwm2m: Fix missing mutex unlock

lwm2m_engine_set() and lwm2m_engine_get() locks
the registry_lock mutex, but this is not unlocked
when setting or getting a time resource where the buffer
lengths are invalid resulting in an early return without
unlocking the mutex. This results in a deadlock when
attempting to lock the registry in another thread.

Signed-off-by: Steven Poon <steven-github@outlook.com>