QTiffHandler: fix Coverity warning about unchecked return values

When TIFFIsTiled() returns true, then TIFFGetField(TILEWIDTH/LENGTH)
oughtn't fail, but don't depend on internals of libtiff (the docs
don't guarantee this), so check their return values, too.

Found by Coverity complaining that two out of 16 TIFFGetField() calls
didn't have their return values checked.

Amends 30301436b59efe7ff25991cbbaa388f6f1b4d66e (5.9).

Pick-to: 6.12 6.11 6.8
Coverity-Id: 178800
Change-Id: I8aa8e2080b47ea0597170a46ddc86699f438d001
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>