return empty string instead of some garbage if strftime() failed

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@54961 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/datetime.cpp

@@ -487,8 +487,10 @@ static wxString CallStrftime(const wxString& format, const tm* tm)
 
     if ( !wxStrftime(buf, WXSIZEOF(buf), format, tm) )
     {
-        // buffer is too small?
+        // if the format is valid, buffer must be too small?
         wxFAIL_MSG(_T("strftime() failed"));
+
+        buf[0] = '\0';
     }
 
     s = buf;