c.f. the constructor already doing the same for provider and the
various fields populated by initControls(), which init() calls.
Should be redundant (all QCamera constructors do invoke d->init() as
soon as they've constructed d), but better safe than sorry.  Calms
Coverity (CID 22319).

Change-Id: Ibf5d13bb1ef17be8e66fa14ed5617b700051c495
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>