patch 7.4.870

Problem:    May get into an invalid state when using getchar() in an
            expression mapping.
Solution:   Anticipate mod_mask to change. (idea by Yukihiro Nakadaira)

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    870,
 /**/
     869,
 /**/