Core/Calendar: Fix memory leak in CMSG_CALENDAR_ADD_EVENT handler.

Fix memory leak when malformed CMSG_CALENDAR_ADD_EVENT is received.
This properly fixes the memory leak that 51e38872c72e16bb0f09e660c2660759021b2176 tried to fix, reverted in 927f16de49d6cee6562e2d6bc48714572d142c03.