Debugger: More robust approach for DebuggerToolTipTreeView::computeSize

model() == 0 should not happen, but the bugreport indicates there
was something wrong at a time.

Task-number: QTCREATORBUG-12692
Change-Id: Ie850dfc1177d968c39ffae74af89ae1bab44703c
Reviewed-by: hjk <hjk121@nokiamail.com>