fnamemodify: fix handling of :r after :e #11165

- Test fnamemodify()
- Test handling of `expand("%:e:e:r")`.
- Fix :e:e:r on filenames with insufficiently many extensions

During `fnamemodify()`, ensuring that we don't go before the filename's
tail is insufficient in cases where we've already handled a ":e"
modifier, for example:

```
"path/to/this.file.ext" :e:e:r:r
         ^    ^-------- *fnamep
         +------------- tail
```

This means for a ":r", we'll go before `*fnamep`, and outside the bounds
of the filename. This is both incorrect and causes neovim to exit with
an allocation error.

We exit because we attempt to calculate `s - *fnamep` (line 23948).
Since `s` is before `*fnamep`, we caluclate a negative length, which
ends up being interpreted as an amount to allocate, causing neovim to
exit with ENOMEM (`memory.c:xmalloc`).

We must instead ensure we don't go before `*fnamep` nor `tail`.
The check for `tail` is still relevant, for example:

```
"path/to/this.file.ext" :r:r:r
 ^       ^------------- tail
 +--------------------- *fnamep
```
Here we don't want to go before `tail`.

close #11165

0 files changed, 0 insertions, 0 deletions