patch 8.2.2357: Vim9: crash when parsing function return type fails

Problem:    Vim9: crash when parsing function return type fails.
Solution:   Bail out and set return type to "unknown". (closes #7685)
diff --git a/src/userfunc.c b/src/userfunc.c
index ded9ef7..7ca8186 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3886,6 +3886,12 @@
 	{
 	    p = ret_type;
 	    fp->uf_ret_type = parse_type(&p, &fp->uf_type_list, TRUE);
+	    if (fp->uf_ret_type == NULL)
+	    {
+		fp->uf_ret_type = &t_void;
+		SOURCING_LNUM = lnum_save;
+		goto erret;
+	    }
 	}
 	SOURCING_LNUM = lnum_save;
     }