Fix dangling pointers

std::vector gives zero guarantees that pointers to its elements remain
valid when the vector's size changes. In particular, pushing new
elements into the vector may trigger reallocation of the underlying heap
area.

Consequently, Event::delegate() needs to ensure that any modifications
to the d->attendees vector are performed before pointers to its elements
are taken and collected.

Found with Valgrind.

Reviewers: mollekopf

Reviewed By: mollekopf

Subscribers: mollekopf

Differential Revision: https://git.kolab.org/D2548