patch 8.2.3779: using freed memory when defining a user command recursively

Problem:    Using freed memory when defining a user command from a user
            command.
Solution:   Do not use the command pointer after executing the command.
            (closes #9318)

src/testdir/test_usercommands.vim

@@ -704,5 +704,24 @@ def Test_count_with_quotes()
   delcommand GetCount
 enddef
 
+func DefCmd(name)
+  if len(a:name) > 30
+    return
+  endif
+  exe 'command ' .. a:name .. ' call DefCmd("' .. a:name .. 'x")'
+  echo a:name
+  exe a:name
+endfunc
+
+func Test_recursive_define()
+  call DefCmd('Command')
+
+  let name = 'Command'
+  while len(name) < 30
+    exe 'delcommand ' .. name
+    let name ..= 'x'
+  endwhile
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab

src/usercmd.c

@@ -1670,7 +1670,8 @@ do_ucmd(exarg_T *eap)
     size_t	split_len = 0;
     char_u	*split_buf = NULL;
     ucmd_T	*cmd;
-    sctx_T	save_current_sctx = current_sctx;
+    sctx_T	save_current_sctx;
+    int		restore_current_sctx = FALSE;
 
     if (eap->cmdidx == CMD_USER)
 	cmd = USER_CMD(eap->useridx);
@@ -1771,14 +1772,20 @@ do_ucmd(exarg_T *eap)
 
     if ((cmd->uc_argt & EX_KEEPSCRIPT) == 0)
     {
+	restore_current_sctx = TRUE;
+	save_current_sctx = current_sctx;
 	current_sctx.sc_version = cmd->uc_script_ctx.sc_version;
 #ifdef FEAT_EVAL
 	current_sctx.sc_sid = cmd->uc_script_ctx.sc_sid;
 #endif
     }
+
     (void)do_cmdline(buf, eap->getline, eap->cookie,
 				   DOCMD_VERBOSE|DOCMD_NOWAIT|DOCMD_KEYTYPED);
-    if ((cmd->uc_argt & EX_KEEPSCRIPT) == 0)
+
+    // Careful: Do not use "cmd" here, it may have become invalid if a user
+    // command was added.
+    if (restore_current_sctx)
 	current_sctx = save_current_sctx;
     vim_free(buf);
     vim_free(split_buf);

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3779,
 /**/
     3778,
 /**/