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/autocmd.c b/src/autocmd.c
index d6945f3..937e741 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -763,7 +763,7 @@
     save_ei = vim_strsave(p_ei);
     if (save_ei != NULL)
     {
-	new_ei = vim_strnsave(p_ei, (int)(STRLEN(p_ei) + STRLEN(what)));
+	new_ei = vim_strnsave(p_ei, STRLEN(p_ei) + STRLEN(what));
 	if (new_ei != NULL)
 	{
 	    if (*what == ',' && *p_ei == NUL)
@@ -991,7 +991,7 @@
 	;
     if (p > arg)
     {
-	group_name = vim_strnsave(arg, (int)(p - arg));
+	group_name = vim_strnsave(arg, p - arg);
 	if (group_name == NULL)		// out of memory
 	    return AUGROUP_ERROR;
 	group = au_find_group(group_name);