cmCursesLongMessageForm: Avoid -Wstringop-overflow warning

We use `strncpy` to copy the title up to a maximum number of
characters.  GCC 8's `-Wstringop-overflow` warns that the length
depends on the input length because it fails to recognize that we
are bounding it to the buffer size too.  Update the code to hide
the dependence on the input length.

Source/CursesDialog/cmCursesLongMessageForm.cxx

@@ -43,7 +43,7 @@ void cmCursesLongMessageForm::UpdateStatusBar()
   getmaxyx(stdscr, y, x);
 
   char bar[cmCursesMainForm::MAX_WIDTH];
-  size_t size = strlen(this->Title.c_str());
+  size_t size = this->Title.size();
   if (size >= cmCursesMainForm::MAX_WIDTH) {
     size = cmCursesMainForm::MAX_WIDTH - 1;
   }