QFINDTESTDATA,Android: fix 'relative to test source'

When cross-compiling for Android on Windows the 'relative to test
source' option could end up possibly matching with a matching folder in
root ('/') because file is a Windows path and the canonicalFilePath of
that path is "".

Fixes tst_qxmlstream (and possibly others) on Android when Qt is
compiled on Windows.

Task-number: QTBUG-68596
Change-Id: I378374e41eea80f43680b3941adaa91fa604934a
Reviewed-by: Jesus Fernandez <[email protected]>
Reviewed-by: Thiago Macieira <[email protected]>
Reviewed-by: Ulf Hermann <[email protected]>

src/testlib/qtestcase.cpp

@@ -2135,8 +2135,9 @@ QString QTest::qFindTestData(const QString& base, const char *file, int line, co
             srcdir.setFile(QFile::decodeName(builddir) + QLatin1String("/") + srcdir.filePath());
         }
 
-        QString candidate = QString::fromLatin1("%1/%2").arg(srcdir.canonicalFilePath(), base);
-        if (QFileInfo::exists(candidate)) {
+        const QString canonicalPath = srcdir.canonicalFilePath();
+        QString candidate = QString::fromLatin1("%1/%2").arg(canonicalPath, base);
+        if (!canonicalPath.isEmpty() && QFileInfo::exists(candidate)) {
             found = candidate;
         }
         else if (QTestLog::verboseLevel() >= 2) {