Suppress vim_strnsave() warning

Warning is occurred at here:
https://github.com/vim/vim/blob/7306d6b1c9528dcfe89eb77aa7ef354ffa9605a8/src/vim9compile.c#L4446

Maybe, it's better to change the prototype of vim_strnsave() than adding
type cast.

src/misc2.c

@@ -1287,7 +1287,7 @@ vim_strsave(char_u *string)
  * shorter.
  */
     char_u *
-vim_strnsave(char_u *string, int len)
+vim_strnsave(char_u *string, size_t len)
 {
     char_u	*p;
 
@@ -1534,7 +1534,7 @@ vim_strsave_up(char_u *string)
  * This uses ASCII lower-to-upper case translation, language independent.
  */
     char_u *
-vim_strnsave_up(char_u *string, int len)
+vim_strnsave_up(char_u *string, size_t len)
 {
     char_u *p1;
 

src/proto/misc2.pro

@@ -32,14 +32,14 @@ void *mem_realloc(void *ptr, size_t size);
 void do_outofmem_msg(size_t size);
 void free_all_mem(void);
 char_u *vim_strsave(char_u *string);
-char_u *vim_strnsave(char_u *string, int len);
+char_u *vim_strnsave(char_u *string, size_t len);
 char_u *vim_memsave(char_u *p, size_t len);
 char_u *vim_strsave_escaped(char_u *string, char_u *esc_chars);
 char_u *vim_strsave_escaped_ext(char_u *string, char_u *esc_chars, int cc, int bsl);
 int csh_like_shell(void);
 char_u *vim_strsave_shellescape(char_u *string, int do_special, int do_newline);
 char_u *vim_strsave_up(char_u *string);
-char_u *vim_strnsave_up(char_u *string, int len);
+char_u *vim_strnsave_up(char_u *string, size_t len);
 void vim_strup(char_u *p);
 char_u *strup_save(char_u *orig);
 char_u *strlow_save(char_u *orig);