Assert on invalid date in wxCalendarCtrl::SetDate() in all ports

This was done in wxMSW and wxQt but not in the native GTK+ nor the
generic version, even though they still asserted when actually trying to
use the invalid parameter later.

Make things more clear and consistent by asserting immediately and also
document the behaviour more clearly.

interface/wx/calctrl.h

@@ -469,7 +469,8 @@ public:
 
         The @a date parameter must be valid and in the currently valid range as
         set by SetDateRange(), otherwise the current date is not changed and
-        the function returns @false.
+        the function returns @false and, additionally, triggers an assertion
+        failure if the date is invalid.
     */
     virtual bool SetDate(const wxDateTime& date);
 

src/generic/calctrlg.cpp

@@ -434,6 +434,8 @@ bool wxGenericCalendarCtrl::EnableMonthChange(bool enable)
 
 bool wxGenericCalendarCtrl::SetDate(const wxDateTime& date)
 {
+    wxCHECK_MSG( date.IsValid(), false, "invalid date" );
+
     bool retval = true;
 
     bool sameMonth = m_date.GetMonth() == date.GetMonth(),

src/gtk/calctrl.cpp

@@ -198,7 +198,9 @@ bool wxGtkCalendarCtrl::EnableMonthChange(bool enable)
 
 bool wxGtkCalendarCtrl::SetDate(const wxDateTime& date)
 {
-    if ( date.IsValid() && !IsInValidRange(date) )
+    wxCHECK_MSG( date.IsValid(), false, "invalid date" );
+
+    if ( !IsInValidRange(date) )
         return false;
 
     g_signal_handlers_block_by_func(m_widget,