patch 8.0.1064: Coverity warns for leaking resource

Problem:    Coverity warns for leaking resource.
Solution:   Free pty_master_fd on failure.
diff --git a/src/os_unix.c b/src/os_unix.c
index 57ea4d9..1ec59fc 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -5692,7 +5692,10 @@
 
     channel = add_channel();
     if (channel == NULL)
+    {
+	close(pty_master_fd);
 	return FAIL;
+    }
     job->jv_channel = channel;  /* ch_refcount was set by add_channel() */
     channel->ch_keep_open = TRUE;