friendlist access (add, delete, ...) causes crashes sometimes

[iphydf]

Based on #956.

---

between those 2 lines m->friendlist may point to old already freed address

c-toxcore/toxcore/Messenger.c

Lines 67 to 73 in 5b14542

	Friend *newfriendlist = (Friend *)realloc(m->friendlist, num * sizeof(Friend));
	if (newfriendlist == nullptr) {
	return -1;
	}
	m->friendlist = newfriendlist;

ToxAV has other threads, so this may be accessed while invalid.

[iphydf]

Toxcore will have full locking and no interleaved access at all once the toxav separation PR is in.