patch 9.0.0470: in :def function all closures in loop get the same variables

Problem:    In a :def function all closures in a loop get the same variables.
Solution:   When in a loop and a closure refers to a variable declared in the
            loop, prepare for making a copy of variables for each closure.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 9755d97..5530056 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3449,8 +3449,14 @@
 	}
 	dfunc->df_varcount = dfunc->df_var_names.ga_len;
 	dfunc->df_has_closure = cctx.ctx_has_closure;
+
 	if (cctx.ctx_outer_used)
+	{
 	    ufunc->uf_flags |= FC_CLOSURE;
+	    if (outer_cctx != NULL)
+		++outer_cctx->ctx_closure_count;
+	}
+
 	ufunc->uf_def_status = UF_COMPILED;
     }