fix: segfault in find_tagfunc_tags #18841

fixes #15221

I tried to reproduce with a test, but failed. The below patch is able to
cause the out of bound access (I verified by adding a check to the
code), but it doesn't seg fault or trigger asan/valgrind errors.

```
diff --git a/src/nvim/testdir/test_tagfunc.vim b/src/nvim/testdir/test_tagfunc.vim
index ffc1d63b9..22828a39f 100644
--- a/src/nvim/testdir/test_tagfunc.vim
+++ b/src/nvim/testdir/test_tagfunc.vim
@@ -117,4 +117,26 @@ func Test_tagfunc_settagstack()
   delfunc Mytagfunc2
 endfunc

+func Test_tagfunc_settagstack_many()
+
+  func Mytagfunc1(pat, flags, info)
+    return [{'name' : 'mytag', 'filename' : 'Xtest', 'cmd' : '1'}]
+  endfunc
+  set tagfunc=Mytagfunc1
+  call writefile([''], 'Xtest')
+
+  for i in range(0,20)
+    let pos = [bufnr()]  + getcurpos()[1:]
+    let newtag = [{'tagname' : 'mytag' + i, 'from' : pos}]
+    call settagstack(1, {'items' : newtag}, 'a')
+    call settagstack(1, {'curidx' : 21})
+  endfor
+
+  tag
+
+  call delete('Xtest')
+  set tagfunc&
+  delfunc Mytagfunc1
+endfunc
```

1 files changed, 10 insertions, 2 deletions

diff --git a/src/nvim/tag.c b/src/nvim/tag.c
index 73b5bb4058..701d2bc697 100644
--- a/src/nvim/tag.c
+++ b/src/nvim/tag.c
@@ -1143,7 +1143,15 @@ static int find_tagfunc_tags(char_u *pat, garray_T *ga, int *match_count, int fl
   typval_T args[4];
   typval_T rettv;
   char_u flagString[4];
-  taggy_T *tag = &curwin->w_tagstack[curwin->w_tagstackidx];
+  taggy_T *tag = NULL;
+
+  if (curwin->w_tagstacklen > 0) {
+    if (curwin->w_tagstackidx == curwin->w_tagstacklen) {
+      tag = &curwin->w_tagstack[curwin->w_tagstackidx - 1];
+    } else {
+      tag = &curwin->w_tagstack[curwin->w_tagstackidx];
+    }
+  }
 
   if (*curbuf->b_p_tfu == NUL) {
     return FAIL;
@@ -1156,7 +1164,7 @@ static int find_tagfunc_tags(char_u *pat, garray_T *ga, int *match_count, int fl
 
   // create 'info' dict argument
   dict_T *const d = tv_dict_alloc_lock(VAR_FIXED);
-  if (tag->user_data != NULL) {
+  if (tag != NULL && tag->user_data != NULL) {
     tv_dict_add_str(d, S_LEN("user_data"), (const char *)tag->user_data);
   }
   if (buf_ffname != NULL) {