Avoid undefined behavior in 'send-process-region' (Bug#52369).

* src/process.c (send_process): Signal an error if the file descriptor
has already been closed.

src/process.c

@@ -6520,6 +6520,9 @@ send_process (Lisp_Object proc, const char *buf, ptrdiff_t len,
 	  /* Send this batch, using one or more write calls.  */
 	  ptrdiff_t written = 0;
 	  int outfd = p->outfd;
+          if (outfd < 0)
+            error ("Output file descriptor of %s is closed",
+                   SDATA (p->name));
 	  eassert (0 <= outfd && outfd < FD_SETSIZE);
 #ifdef DATAGRAM_SOCKETS
 	  if (DATAGRAM_CHAN_P (outfd))