FilePath: Fix setFromString root for partial path

Change-Id: I3a9352a364c71a8463736b25086a0493ae5c5d31
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/filepath.cpp

@@ -1173,6 +1173,9 @@ void FilePath::setFromString(const QString &filename)
 
                     return;
                 }
+
+                m_data = "/";
+
             } else {
                 m_scheme.clear();
                 m_host.clear();