Cygwin: ptsname_r: always return an error number on failure

Return EBADF on a bad file descriptor.  Previously 0 was returned, in
violation of the requirement in
https://man7.org/linux/man-pages/man3/ptsname_r.3.html that an error
number should be returned on failure.

We are intentionally deviating from Linux, on which ENOTTY is
returned.

Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00245.html

winsup/cygwin/release/3.2.0

@@ -52,3 +52,6 @@ Bug Fixes
 - Fix the errno when a path contains .. and the prefix exists but is
   not a directory.
   Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00214.html
+
+- Fix the return value when ptsname_r(3) is called with a bad file descriptor
+  Addresses: https://lists.gnu.org/archive/html/bug-gnulib/2021-01/msg00245.html

winsup/cygwin/syscalls.cc

@@ -3392,7 +3392,7 @@ ptsname_r (int fd, char *buf, size_t buflen)
 
   cygheap_fdget cfd (fd);
   if (cfd < 0)
-    return 0;
+    return EBADF;
   return cfd->ptsname_r (buf, buflen);
 }