patch 8.0.0448: some macros are in lower case

Problem:    Some macros are in lower case, which can be confusing.
Solution:   Make a few lower case macros upper case.
diff --git a/src/popupmnu.c b/src/popupmnu.c
index 307dbbe..fc5e1c3 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -590,7 +590,7 @@
 			&& curbuf->b_p_bh[0] == 'w')
 		{
 		    /* Already a "wipeout" buffer, make it empty. */
-		    while (!bufempty())
+		    while (!BUFEMPTY())
 			ml_delete((linenr_T)1, FALSE);
 		}
 		else