Fixed tst_qfileinfo::group crash if getgrgid fails

QVERIFY2 and gracefully fail with an error message, rather than
crashing, if getgrgid() fails.

Change-Id: I82a7290f83208486577988cc831d5d3cba20f98e
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>

tests/auto/corelib/io/qfileinfo/tst_qfileinfo.cpp

@@ -1722,7 +1722,13 @@ void tst_QFileInfo::group()
 #if defined(Q_OS_UNIX)
     struct group *gr;
     gid_t gid = getegid();
+
+    errno = 0;
     gr = getgrgid(gid);
+
+    QVERIFY2(gr, qPrintable(
+        QString("getgrgid returned 0: %1, cannot determine my own group")
+        .arg(QString::fromLocal8Bit(strerror(errno)))));
     expected = QString::fromLocal8Bit(gr->gr_name);
 #endif