fix default of SettingsPath when qt.conf is present

it's documented to fall back to Prefix if qt.conf is present but
Settings is not specified.

Task-number: QTBUG-44644
Change-Id: I8ef6659cbdad51b2fb3c1075ea6f0af4997117ed
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qlibraryinfo.cpp

@@ -441,8 +441,10 @@ QLibraryInfo::rawLocation(LibraryLocation loc, PathGroup group)
             defaultValue = QLatin1String(qtConfEntries[loc].value);
         }
 #ifndef Q_OS_WIN // On Windows we use the registry
-        else if (loc == SettingsPath)
+        else if (loc == SettingsPath) {
             key = QLatin1String("Settings");
+            defaultValue = QLatin1String(".");
+        }
 #endif
 
         if(!key.isNull()) {