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
```

0 files changed, 0 insertions, 0 deletions