QQmlJS::Codegen: Short circuit in qmlErrors to avoid QUrl allocation costs.

This takes the time taken in qmlErrors for my (admittedly terribly morbid)
testcase from ~104ms to ~1ms.

Change-Id: I288086caa6e6b58f67e9feb6f1761c3310f01ead
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>