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/eval.c b/src/eval.c
index b184264..6ca3628 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3139,7 +3139,7 @@
 		    if (n1 >= len || n2 < 0 || n1 > n2)
 			s = NULL;
 		    else
-			s = vim_strnsave(s + n1, (int)(n2 - n1 + 1));
+			s = vim_strnsave(s + n1, n2 - n1 + 1);
 		}
 		else
 		{