RecentModel: prevent nullptr access

Fix a regression introduced by:
[59f41f8d44: avoid adding entry in RecentModel after an incoming ContactRequest]

This patch check if the pointer is not null before using it.

Change-Id: I49b63f4044bbacc06b36e2c10a3fe7af29100477
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>