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/mark.c

@@ -86,7 +86,7 @@ void clear_fmark(fmark_T *fm)
   FUNC_ATTR_NONNULL_ALL
 {
   free_fmark(*fm);
-  memset(fm, 0, sizeof(*fm));
+  CLEAR_POINTER(fm);
 }
 
 /*
@@ -1741,7 +1741,7 @@ void free_all_marks(void)
       free_xfmark(namedfm[i]);
     }
   }
-  memset(&namedfm[0], 0, sizeof(namedfm));
+  CLEAR_FIELD(namedfm);
 }
 #endif