patch 8.2.4269: Coverity warns for using a NULL pointer

Problem:    Coverity warns for using a NULL pointer.
Solution:   Check for "name" to not be NULL.
diff --git a/src/userfunc.c b/src/userfunc.c
index 747e4d4..371cfd3 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -4232,7 +4232,8 @@
 		name = prefixed;
 	    }
 	}
-	else if (vim9script && vim_strchr(name, AUTOLOAD_CHAR) != NULL)
+	else if (vim9script && name != NULL
+				    && vim_strchr(name, AUTOLOAD_CHAR) != NULL)
 	{
 	    emsg(_(e_cannot_use_name_with_hash_in_vim9_script_use_export_instead));
 	    goto ret_free;