QHeaderView - do not ignore -1 as minimum section size

We claim that -1 is a special automatic value, but calling
setMinimumSectionSize with -1 is unfortunately ignored due a
regression in 524c3e05e811d81fbcd71eedb377b7eaa0a21ef3

Change-Id: I7d7e5dbbf78e561849d2f2352c9edb2df36aa181
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>