libvisual/lv_plugin.c: Address warning -Wformat-truncation

Symptom was:
> ../../libvisual-0.4.2/libvisual/lv_plugin.c: In function 'plugin_add_dir_to_list':
> ../../libvisual-0.4.2/libvisual/lv_plugin.c:605:39: error: '%s' directive output may be truncated writing up to 1023 bytes into a region of size 1022 [-Werror=format-truncation=]
>   605 |                 snprintf (temp, 1023, "%s/%s", dir, namelist[i]->d_name);
>       |                                       ^~~~~~~
> ../../libvisual-0.4.2/libvisual/lv_plugin.c:605:43: note: format string is defined here
>   605 |                 snprintf (temp, 1023, "%s/%s", dir, namelist[i]->d_name);
>       |                                           ^~
> ../../libvisual-0.4.2/libvisual/lv_plugin.c:605:17: note: '__builtin_snprintf' output 2 or more bytes (assuming 1025) into a destination of size 1023
>   605 |                 snprintf (temp, 1023, "%s/%s", dir, namelist[i]->d_name);
>       |                 ^~~~~~~~

libvisual/libvisual/lv_plugin.c

@@ -562,7 +562,7 @@ static int plugin_add_dir_to_list (VisList *list, const char *dir)
 
 		if (!(FileData.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY)) {
 
-			snprintf (temp, 1023, "%s\\%s", dir, FileData.cFileName);
+			snprintf (temp, sizeof (temp), "%s\\%s", dir, FileData.cFileName);
 
 			len = strlen (temp);
 			if (len > 5 && (strncmp (&temp[len - 5], ".dll", 5) == 0))
@@ -602,7 +602,7 @@ static int plugin_add_dir_to_list (VisList *list, const char *dir)
 	for (i = 2; i < n; i++) {
 		ref = NULL;
 
-		snprintf (temp, 1023, "%s/%s", dir, namelist[i]->d_name);
+		snprintf (temp, sizeof (temp), "%s/%s", dir, namelist[i]->d_name);
 
 		len = strlen (temp);
 		if (len > 3 && (strncmp (&temp[len - 3], ".so", 3) == 0))