patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
diff --git a/src/netbeans.c b/src/netbeans.c
index 4d79a2e..4ff9085 100644
--- a/src/netbeans.c
+++ b/src/netbeans.c
@@ -1547,7 +1547,7 @@
 		    // disappear.
 		    do_bufdel(DOBUF_DEL, (char_u *)"", 1,
 				  buf->bufp->b_fnum, buf->bufp->b_fnum, TRUE);
-		    vim_memset(buf, 0, sizeof(nbbuf_T));
+		    CLEAR_POINTER(buf);
 		}
 	    }
 // =====================================================================