patch 8.2.4082: check for autoload file name and prefix fails

Problem:    Check for autoload file name and prefix fails. (Christian J.
            Robinson)
Solution:   Only lower case the prefix on systems where the file name is not
            case sensitive.

src/scriptfile.c

@@ -2156,7 +2156,11 @@ get_autoload_prefix(scriptitem_T *si)
 
     if (p == NULL)
 	return NULL;
+#ifdef CASE_INSENSITIVE_FILENAME
     prefix = strlow_save(p);
+#else
+    prefix = vim_strsave(p);
+#endif
     if (prefix == NULL)
 	return NULL;