Cygwin: path_conv: do not get confused by a directory with `.lnk` suffix

When trying to create a directory called `xyz` in the presence of a
directory `xyz.lnk`, the Cygwin runtime errors out with an `ENOENT`.

The root cause is actually a bit deeper: the `symlink_info::check()`
method tries to figure out whether the given path refers to a symbolic
link as emulated via `.lnk` files, but since it is a directory, that is
not the case, and that hypothesis is rejected.

However, the `fileattr` field is not cleared, so that a later
`.exists()` call on the instance mistakenly thinks that the symlink
actually exists. Let's clear that field.

This fixes https://github.com/msys2/msys2-runtime/issues/81

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/path.cc

@@ -3336,6 +3336,7 @@ restart:
 	 hasn't been found. */
       if (ext_tacked_on && !had_ext && (fileattr & FILE_ATTRIBUTE_DIRECTORY))
 	{
+	  fileattr = INVALID_FILE_ATTRIBUTES;
 	  set_error (ENOENT);
 	  continue;
 	}