Harden tiff handler

- Avoid uninitialized data in resulting QImage, for truncated
  scanlines or tiles in the direct-read decoding path.

- Fix wrong assert in rgbFixup(): Can be called for RGBX64 also, not
  only FP formats. Ensure that the stored alpha value is meaningful in
  that case.

- Explicitly detect and reject separate-plane files on the direct-read
  path, which is not prepared to handle them correctly.

- When writing, guard against oob when input color table is empty

- When writing, avoid uninitialized data in the resulting color tables.

Task-number: QTBUG-148329
Pick-to: 6.12 6.11 6.8 6.5 5.15
Change-Id: Id6e3074f55345cffec7819f8f45d07814a8f7c31
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>