QmlProfiler: Don't try to paint things in pixmap of negative size.

In certain conditions the area covered by the canvas can have a negative
size. This patch handles that case gracefully.

Change-Id: I952edfa5bd0f8b42ce69356106f2e0f326dceeb7
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/plugins/qmlprofiler/canvas/qmlprofilercanvas.cpp

@@ -69,7 +69,7 @@ void QmlProfilerCanvas::draw()
     m_context2d->reset();
     m_context2d->setSize(width(), height());
 
-    if (width() != 0 && height() != 0)
+    if (width() > 0 && height() > 0)
         emit drawRegion(m_context2d, QRect(0, 0, width(), height()));
     update();
 }