Check for end of string when parsing "%z" in wxDateTime::Format().

We could dereference an invalid iterator when parsing "%z", check for this in
the code and add a unit test for this case.

Closes #14075.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70846 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/datetime/datetimetest.cpp

@@ -829,6 +829,9 @@ void DateTimeTestCase::TestTimeFormat()
     CPPUNIT_ASSERT( dt.ParseFormat("17", "%d") );
     CPPUNIT_ASSERT_EQUAL( 17, dt.GetDay() );
 
+    // test some degenerate cases
+    CPPUNIT_ASSERT( !dt.ParseFormat("", "%z") );
+
     // test compilation of some calls which should compile (and not result in
     // ambiguity because of char*<->wxCStrData<->wxString conversions)
     wxString s("foo");