patch 8.2.3134: crash when using typename() on a function reference

Problem:    Crash when using typename() on a function reference. (Naohiro Ono)
Solution:   Initialize pointer to NULL. (closes #8531)

src/testdir/test_vim9_builtin.vim

@@ -1943,6 +1943,12 @@ def Test_tr()
   CheckDefFailure(['echo tr("a", "a", 1)'], 'E1013: Argument 3: type mismatch, expected string but got number')
 enddef
 
+def Test_typename()
+  if has('float')
+    assert_equal('func([unknown], [unknown]): float', typename(function('pow')))
+  endif
+enddef
+
 def Test_undofile()
   CheckDefFailure(['undofile(10)'], 'E1013: Argument 1: type mismatch, expected string but got number')
   assert_equal('.abc.un~', fnamemodify(undofile('abc'), ':t'))

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3134,
 /**/
     3133,
 /**/

src/vim9type.c

@@ -1166,7 +1166,7 @@ type_name(type_T *type, char **tofree)
 
 	for (i = 0; i < type->tt_argcount; ++i)
 	{
-	    char *arg_free;
+	    char *arg_free = NULL;
 	    char *arg_type;
 	    int  len;