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>
diff --git a/src/misc1.c b/src/misc1.c
index bf70e38..3085fc7 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -1404,9 +1404,6 @@
     int		mustfree;	// var was allocated, need to free it later
     int		at_start = TRUE; // at start of a name
     int		startstr_len = 0;
-#if defined(BACKSLASH_IN_FILENAME) || defined(AMIGA)
-    char_u	*save_dst = dst;
-#endif
 
     if (startstr != NULL)
 	startstr_len = (int)STRLEN(startstr);
@@ -1631,7 +1628,7 @@
 		// with it, skip a character
 		if (after_pathsep(dst, dst + c)
 #if defined(BACKSLASH_IN_FILENAME) || defined(AMIGA)
-			&& (dst == save_dst || dst[-1] != ':')
+			&& dst[c - 1] != ':'
 #endif
 			&& vim_ispathsep(*tail))
 		    ++tail;