refactor: use CLEAR_FIELD and CLEAR_POINTER macros (#19709)

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
a80faa8930

src/nvim/main.c

@@ -1418,7 +1418,7 @@ scripterror:
  * copied, so that they can be changed. */
 static void init_params(mparm_T *paramp, int argc, char **argv)
 {
-  memset(paramp, 0, sizeof(*paramp));
+  CLEAR_POINTER(paramp);
   paramp->argc = argc;
   paramp->argv = argv;
   paramp->use_debug_break_level = -1;