Fix tab icon vertical alignment

Fixes case if custom icon size is given and height is more than width.
Check of minimum valid height was compared wrongly to width and caused invalid
vertical alignment. After fix vertical alignment is correct aligned to vertical
center with the tab text.

Change-Id: I6c4a710b15e91225edeabb629bfea7049ab2f42a
Task-number: QTBUG-42143
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Jens Bache-Wiig <jensbw@gmail.com>