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/gui_motif.c b/src/gui_motif.c
index 28b080a..022626b 100644
--- a/src/gui_motif.c
+++ b/src/gui_motif.c
@@ -1956,7 +1956,7 @@
 
 		    XmProcessTraversal(w, XmTRAVERSE_CURRENT);
 
-		    vim_memset((char *) &keyEvent, 0, sizeof(XKeyPressedEvent));
+		    CLEAR_FIELD(keyEvent);
 		    keyEvent.type = KeyPress;
 		    keyEvent.serial = 1;
 		    keyEvent.send_event = True;