patch 8.2.0928: many type casts are used for vim_strnsave()

Problem:    Many type casts are used for vim_strnsave().
Solution:   Make the length argument size_t instead of int. (Ken Takata,
            closes #5633)  Remove some type casts.
diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index f84bf7a..1e33987 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -2555,7 +2555,7 @@
 	{
 	    if (ga_grow(&ga, 1) == FAIL)
 		break;
-	    ((char_u **)ga.ga_data)[ga.ga_len] = vim_strnsave(s, (int)(e - s));
+	    ((char_u **)ga.ga_data)[ga.ga_len] = vim_strnsave(s, e - s);
 	    ++ga.ga_len;
 	}