patch 9.1.0161: expand() removes slash after env variable that ends with colon

patch 9.1.0161: expand() removes slash after env variable that ends with colon

Problem:  expand() removes a slash after an environment variable that
          ends with a colon on Windows.
Solution: Check the correct char for a colon (zeertzjq)

closes: #14161

Note: Vim still removes the path-separator at the end, if another path separator
follows directly after it, e.g. on:

```
    echo $FOO='/usr/'
    echo expand('$FOO/bar') == '/usr/bar'
```

see:

,----[ misc1.c:1630 ]
|   // if var[] ends in a path separator and tail[] starts
|   // with it, skip a character
|   if (after_pathsep(dst, dst + c)
| #if defined(BACKSLASH_IN_FILENAME) || defined(AMIGA)
| 	  && (dst == save_dst || dst[-1] != ':')
| #endif
| 	  && vim_ispathsep(*tail))
|       ++tail;
`----

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>