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/syntax.c b/src/syntax.c
index f2f74a1..5c9dd0b 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -5097,7 +5097,7 @@
 	while (*key_end && !VIM_ISWHITE(*key_end) && *key_end != '=')
 	    ++key_end;
 	vim_free(key);
-	key = vim_strnsave_up(rest, (int)(key_end - rest));
+	key = vim_strnsave_up(rest, key_end - rest);
 	if (key == NULL)			// out of memory
 	{
 	    rest = NULL;
@@ -5762,7 +5762,7 @@
 	arg_end = skiptowhite(arg_start);
 	next_arg = skipwhite(arg_end);
 	vim_free(key);
-	key = vim_strnsave_up(arg_start, (int)(arg_end - arg_start));
+	key = vim_strnsave_up(arg_start, arg_end - arg_start);
 	if (STRCMP(key, "CCOMMENT") == 0)
 	{
 	    if (!eap->skip)