fix(treesitter): crash in ts_parser_delete after gc #39497

Problem:
parser_gc() calls ts_parser_delete() but leaves the userdata pointer
pointing to freed memory. If the GC finalizer runs at an unexpected time
(e.g. inside nvim_buf_get_lines #39411), a stale pointer could cause a crash.

Solution:
- NULL out `*ud` after ts_parser_delete() in parser_gc()
- Update parser_check() to handle NULL with a clear error message,
guarding all parser methods against UAF

Co-authored-by: Lewis Russell <lewis6991@gmail.com>
Signed-off-by: Szymon Wilczek <swilczek.lx@gmail.com>
(cherry picked from commit 0c3e6e1b0e)

src/nvim/lua/treesitter.c

@@ -400,10 +400,10 @@ static int tslua_push_parser(lua_State *L)
   return 1;
 }
 
-static TSParser *parser_check(lua_State *L, uint16_t index)
+static TSParser *parser_check(lua_State *L, int index)
 {
   TSParser **ud = luaL_checkudata(L, index, TS_META_PARSER);
-  luaL_argcheck(L, *ud, index, "TSParser expected");
+  luaL_argcheck(L, *ud != NULL, index, "Parser has been deleted");
   return *ud;
 }
 
@@ -420,9 +420,12 @@ static void logger_gc(TSLogger logger)
 
 static int parser_gc(lua_State *L)
 {
-  TSParser *p = parser_check(L, 1);
-  logger_gc(ts_parser_logger(p));
-  ts_parser_delete(p);
+  TSParser **ud = luaL_checkudata(L, 1, TS_META_PARSER);
+  if (*ud) {
+    logger_gc(ts_parser_logger(*ud));
+    ts_parser_delete(*ud);
+    *ud = NULL;
+  }
   return 0;
 }