Please note that emsmdbp_get_mailbox() is not intended to return NULL.
At the moment, if we pass a NULL or object without parent mailbox, it is going to segfault.

I intended to make a separate patch that enables emsmdbp_get_mailbox() to return NULLs
+ all callers to check return value.
Although, thinking of it - we should not have ever pass a NULL to emsmdbp_get_mailbox()
neither we should have objects without associated 'mailbox' parent object.
Thus segfault-ing or OC_ABORT() in emsmdbp_get_mailbox() is the right way to go :-)