Cygwin: is_console_app(): do handle errors

When that function was introduced in bb42852 (Cygwin: pty: Implement
new pseudo console support., 2020-08-19) (back then, it was added to
`spawn.cc`, later it was moved to `fhandler/termios.cc` in 32d6a6c
(Cygwin: pty, console: Encapsulate spawn.cc code related to
pty/console., 2022-11-19)), it was implemented with strong assumptions
that neither creating the file handle nor reading 1024 bytes from said
handle could fail.

This assumption, however, is incorrect. Concretely, I encountered the
case where `is_console_app()` needed to open an app execution alias,
failed to do so, and still tried to read from the invalid handle.

Let's add some error handling to that function.

Fixes: bb42852 (Cygwin: pty: Implement new pseudo console support., 2020-08-19)
Co-authored-by: Takashi Yano <takashi.yano@nifty.ne.jp>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
(cherry picked from commit c9b98d2e553b5d3c08b597ec1c84b7ccf0a249fe)

Author: dscho

winsup/cygwin/fhandler/termios.cc

@@ -707,10 +707,14 @@ is_console_app (const WCHAR *filename)
   HANDLE h;
   h = CreateFileW (filename, GENERIC_READ, FILE_SHARE_READ,
 		   NULL, OPEN_EXISTING, 0, NULL);
+  if (h == INVALID_HANDLE_VALUE)
+    return true;
   char buf[1024];
   DWORD n;
-  ReadFile (h, buf, sizeof (buf), &n, 0);
+  BOOL res = ReadFile (h, buf, sizeof (buf), &n, 0);
   CloseHandle (h);
+  if (!res)
+    return true;
   /* The offset of Subsystem is the same for both IMAGE_NT_HEADERS32 and
      IMAGE_NT_HEADERS64, so only IMAGE_NT_HEADERS32 is used here. */
   IMAGE_NT_HEADERS32 *p = (IMAGE_NT_HEADERS32 *) memmem (buf, n, "PE\0\0", 4);