patch 8.2.4148: deleting any mapping may cause <ScritpCmd> to fail

Problem:    Deleting any mapping may cause <ScritpCmd> to not set the script
            context.
Solution:   Only reset last_used_map if it is the deleted mapping.
            (closes #9568)

src/getchar.c

@@ -3836,8 +3836,9 @@ do_cmdkey_command(int key UNUSED, int flags)
 
 #if defined(FEAT_EVAL) || defined(PROTO)
     void
-reset_last_used_map(void)
+reset_last_used_map(mapblock_T *mp)
 {
+    if (last_used_map == mp)
 	last_used_map = NULL;
 }
 #endif

src/map.c

@@ -86,7 +86,7 @@ map_free(mapblock_T **mpp)
     *mpp = mp->m_next;
     vim_free(mp);
 #ifdef FEAT_EVAL
-    reset_last_used_map();
+    reset_last_used_map(mp);
 #endif
 }
 

src/proto/getchar.pro

@@ -53,5 +53,5 @@ void vungetc(int c);
 int fix_input_buffer(char_u *buf, int len);
 int input_available(void);
 int do_cmdkey_command(int key, int flags);
-void reset_last_used_map(void);
+void reset_last_used_map(mapblock_T *mp);
 /* vim: set ft=c : */

src/testdir/test_mapping.vim

@@ -1439,6 +1439,22 @@ func Test_map_script_cmd_finds_func()
   unlet g:func_called
 endfunc
 
+func Test_map_script_cmd_survives_unmap()
+  let lines =<< trim END
+      vim9script
+      var n = 123
+      nnoremap <F4> <ScriptCmd><CR>
+      autocmd CmdlineEnter * silent! nunmap <F4>
+      nnoremap <F3> :<ScriptCmd>eval setbufvar(bufnr(), "result", n)<CR>
+      feedkeys("\<F3>\<CR>", 'xct')
+      assert_equal(123, b:result)
+  END
+  call CheckScriptSuccess(lines)
+
+  nunmap <F3>
+  unlet b:result
+endfunc
+
 " Test for using <script> with a map to remap characters in rhs
 func Test_script_local_remap()
   new

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4148,
 /**/
     4147,
 /**/