Cygwin: fix handling of known reparse points that are not symlinks

Commit aa467e6e, "Cygwin: add AF_UNIX reparse points to path
handling", changed check_reparse_point_target so that it could return
a positive value on a known reparse point that is not a symlink.  But
some of the code in check_reparse_point that handles this positive
return value was executed unconditionally, when it should have been
executed only for symlinks.

As a result, posixify could be called on a buffer containing garbage,
and check_reparse_point could erroneously return a positive value on a
non-symlink.  This is now fixed so that posixify is only called if the
reparse point is a symlink, and check_reparse_point returns 0 if the
reparse point is not a symlink.

Also fix symlink_info::check to handle this last case, in which
check_reparse_point returns 0 on a known reparse point.

winsup/cygwin/path.cc

@@ -2655,11 +2655,15 @@ symlink_info::check_reparse_point (HANDLE h, bool remote)
   /* ret is > 0, so it's a known reparse point, path in symbuf. */
   path_flags |= ret;
   if (ret & PATH_SYMLINK)
-    sys_wcstombs (srcbuf, SYMLINK_MAX + 7, symbuf.Buffer,
-		  symbuf.Length / sizeof (WCHAR));
-  /* A symlink is never a directory. */
-  fileattr &= ~FILE_ATTRIBUTE_DIRECTORY;
-  return posixify (srcbuf);
+    {
+      sys_wcstombs (srcbuf, SYMLINK_MAX + 7, symbuf.Buffer,
+		    symbuf.Length / sizeof (WCHAR));
+      /* A symlink is never a directory. */
+      fileattr &= ~FILE_ATTRIBUTE_DIRECTORY;
+      return posixify (srcbuf);
+    }
+  else
+    return 0;
 }
 
 int
@@ -3274,6 +3278,9 @@ restart:
 		&= ~FILE_ATTRIBUTE_DIRECTORY;
 	      break;
 	    }
+	  else if (res == 0 && (path_flags & PATH_REP))
+	    /* Known reparse point but not a symlink. */
+	    goto file_not_symlink;
 	  else
 	    {
 	      /* Volume moint point or unrecognized reparse point type.