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