patch 8.1.2120: some MB_ macros are more complicated than necessary

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.
diff --git a/src/getchar.c b/src/getchar.c
index 5e098df..85cedde 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2254,7 +2254,7 @@
 		    char_u *p2 = mb_unescape(&p1);
 
 		    if (has_mbyte && p2 != NULL
-					&& MB_BYTE2LEN(tb_c1) > MB_PTR2LEN(p2))
+					&& MB_BYTE2LEN(tb_c1) > mb_ptr2len(p2))
 			mlen = 0;
 		}