patch 8.0.0980: Coverity warning for failing to open /dev/null

Problem:    Coverity warning for failing to open /dev/null.
Solution:   When /dev/null can't be opened exit the child.

Author: brammool

src/os_unix.c

@@ -4340,6 +4340,7 @@ mch_call_shell(
 
 # define EXEC_FAILED 122    /* Exit code when shell didn't execute.  Don't use
 			       127, some shells use that already */
+# define OPEN_NULL_FAILED 123 /* Exit code if /dev/null can't be opened */
 
     char_u	*newcmd;
     pid_t	pid;
@@ -5369,7 +5370,14 @@ mch_job_start(char **argv, job_T *job, jobopt_T *options)
 	}
 
 	if (use_null_for_in || use_null_for_out || use_null_for_err)
+	{
 	    null_fd = open("/dev/null", O_RDWR | O_EXTRA, 0);
+	    if (null_fd < 0)
+	    {
+		perror("opening /dev/null failed");
+		_exit(OPEN_NULL_FAILED);
+	    }
+	}
 
 	if (pty_slave_fd >= 0)
 	{

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    980,
 /**/
     979,
 /**/