- 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
