patch 8.1.1968: crash when using nested map()

Problem:    Crash when using nested map().
Solution:   Clear the pointer in prepare_vimvar(). (Ozaki Kiichi,
            closes #4890, closes #4891)

src/evalvars.c

@@ -489,19 +489,21 @@ get_spellword(list_T *list, char_u **pp)
 
 /*
  * Prepare v: variable "idx" to be used.
- * Save the current typeval in "save_tv".
+ * Save the current typeval in "save_tv" and clear it.
  * When not used yet add the variable to the v: hashtable.
  */
     void
 prepare_vimvar(int idx, typval_T *save_tv)
 {
     *save_tv = vimvars[idx].vv_tv;
+    vimvars[idx].vv_str = NULL;  // don't free it now
     if (vimvars[idx].vv_type == VAR_UNKNOWN)
 	hash_add(&vimvarht, vimvars[idx].vv_di.di_key);
 }
 
 /*
  * Restore v: variable "idx" to typeval "save_tv".
+ * Note that the v: variable must have been cleared already.
  * When no longer defined, remove the variable from the v: hashtable.
  */
     void

src/testdir/test_filter_map.vim

@@ -57,6 +57,9 @@ func Test_filter_map_nested()
   let x = {"x":10}
   let r = map(range(2), 'filter(copy(x), "1")')
   call assert_equal([x, x], r)
+
+  let r = map(copy(x), 'filter(copy(x), "1")')
+  call assert_equal({"x": x}, r)
 endfunc
 
 " dict with funcref

src/testdir/test_functions.vim

@@ -1543,6 +1543,10 @@ func Test_readdir()
   let files = readdir('Xdir', {x -> len(add(l, x)) == 2 ? -1 : 1})
   call assert_equal(1, len(files))
 
+  " Nested readdir() must not crash
+  let files = readdir('Xdir', 'readdir("Xdir", "1") != []')
+  call sort(files)->assert_equal(['bar.txt', 'dir', 'foo.txt'])
+
   eval 'Xdir'->delete('rf')
 endfunc
 

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1968,
 /**/
     1967,
 /**/