Don't crash when input is empty in wxFileConfig(wxInputStream) ctor.

Fix crash due to dereferencing a NULL pointer when the input buffer in
wxFileConfig ctor is empty.

Closes #11636.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63228 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/config/fileconf.cpp

@@ -81,6 +81,7 @@ private:
         CPPUNIT_TEST( DeleteAndRecreateGroup );
         CPPUNIT_TEST( AddToExistingRoot );
         CPPUNIT_TEST( ReadNonExistent );
+        CPPUNIT_TEST( ReadEmpty );
     CPPUNIT_TEST_SUITE_END();
 
     void Path();
@@ -103,6 +104,7 @@ private:
     void DeleteAndRecreateGroup();
     void AddToExistingRoot();
     void ReadNonExistent();
+    void ReadEmpty();
 
 
     static wxString ChangePath(wxFileConfig& fc, const wxChar *path)
@@ -649,5 +651,13 @@ void FileConfigTestCase::ReadNonExistent()
     CPPUNIT_ASSERT( !fc.Read("URL", &url) );
 }
 
+void FileConfigTestCase::ReadEmpty()
+{
+    static const char *confTest = "";
+
+    wxStringInputStream sis(confTest);
+    wxFileConfig fc(sis);
+}
+
 #endif // wxUSE_FILECONFIG