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

src/common/datetimefmt.cpp

@@ -1408,6 +1408,11 @@ wxDateTime::ParseFormat(const wxString& date,
 
             case wxT('z'):
                 {
+                    // check that we have something here at all
+                    if ( input == end )
+                        return false;
+
+                    // and then check that it's either plus or minus sign
                     bool minusFound;
                     if ( *input == wxT('-') )
                         minusFound = true;