patch 8.2.3771: Vim9: accessing freed memory when checking type

Problem:    Vim9: accessing freed memory when checking type.
Solution:   Make a copy of a function type.

src/testdir/test_vim9_func.vim

@@ -1224,6 +1224,25 @@ def Test_set_opfunc_to_lambda()
   CheckScriptSuccess(lines)
 enddef
 
+def Test_lambda_type_allocated()
+  # Check that unreferencing a partial using a lambda can use the variable type
+  # after the lambda has been freed and does not leak memory.
+  var lines =<< trim END
+    vim9script
+
+    func MyomniFunc1(val, findstart, base)
+      return a:findstart ? 0 : []
+    endfunc
+
+    var Lambda = (a, b) => MyomniFunc1(19, a, b)
+    &omnifunc = Lambda
+    Lambda = (a, b) => MyomniFunc1(20, a, b)
+    &omnifunc = string(Lambda)
+    Lambda = (a, b) => strlen(a)
+  END
+  CheckScriptSuccess(lines)
+enddef
+
 " Default arg and varargs
 def MyDefVarargs(one: string, two = 'foo', ...rest: list<string>): string
   var res = one .. ',' .. two