avoid that a bad qmakespec path crashes the evaluator

assigning a relative path to QMAKESPEC or QMAKESPEC_ORIGINAL (in the qt4
windows legacy code) would lead to an assert further down the line.
just ignore such attempts silently.

Task-number: QTCREATORBUG-8477
Change-Id: Ie53d0ef004c743284b85de4e89f112e0161ff4b7
Reviewed-by: Daniel Teske <daniel.teske@digia.com>