QTiffHandler: fix missing member init

Coverity complained that the Private ctor left the floatingPoint
member uninitialized. At the moment, this is harmless, since all
readers are gated on headersRead == true, which is where floatingPoint
is also set unconditionally, but this is of course brittle, and the
similar grayscale member is also initialized. So initialize
floatingPoint, too.

Amends 95f02ee91fea543ab313253c867bcfb71da60c5e (6.0).

Picking to all active branches, since it's totally safe, even if it's
not required.

Pick-to: 6.12 6.11 6.8
Coverity-Id: 385005
Change-Id: Ie547d1853d7e1e726c713176a9042b7fd974ec07
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>