patch 8.2.0279: Vim9: no test for deleted :def function

Problem:    Vim9: no test for deleted :def function.
Solution:   Add a test.  Clear uf_cleared flag when redefining a function.
diff --git a/src/userfunc.c b/src/userfunc.c
index fc2161f..9ff78c2 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3053,6 +3053,7 @@
 	flags |= FC_SANDBOX;
     fp->uf_flags = flags;
     fp->uf_calls = 0;
+    fp->uf_cleared = FALSE;
     fp->uf_script_ctx = current_sctx;
     fp->uf_script_ctx.sc_lnum += sourcing_lnum_top;
     if (is_export)