patch 8.2.2764: memory leak when default function argument is allocated

Problem:    Memory leak when default function argument is allocated.
Solution:   Free the expression result.

src/testdir/test_functions.vim

@@ -1497,6 +1497,7 @@ endfunc
 
 func Test_balloon_show()
   CheckFeature balloon_eval
+
   " This won't do anything but must not crash either.
   call balloon_show('hi!')
   if !has('gui_running')
@@ -2650,4 +2651,12 @@ func Test_browsedir()
   call assert_fails('call browsedir("open", [])', 'E730:')
 endfunc
 
+func HasDefault(msg = 'msg')
+  return a:msg
+endfunc
+
+func Test_default_arg_value()
+  call assert_equal('msg', HasDefault())
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab