QPicture: fix crash for malformed picture

A file with the correct QPicture magic bytes, but shorter than a full
QPicture file header, could cause the header decoder to access memory
out of bounds. Add a size check to avoid.

As a driveby, generally harden the parsing against malformed files.

[ChangeLog][QtGui][QPicture] Fix crash reading malformed picture file

Task-number: QTBUG-71208
Change-Id: I86eb1f915ca9b3a4b91c7433036d76ed6061e2f0
Reviewed-by: Lars Knoll <lars.knoll@qt.io>