run-command: dup_devnull(): guard against syscalls failing

dup_devnull() did not check the return values of open() and dup2().
Fix this omission.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -76,7 +76,10 @@ static inline void close_pair(int fd[2])
 static inline void dup_devnull(int to)
 {
 	int fd = open("/dev/null", O_RDWR);
-	dup2(fd, to);
+	if (fd < 0)
+		die_errno(_("open /dev/null failed"));
+	if (dup2(fd, to) < 0)
+		die_errno(_("dup2(%d,%d) failed"), fd, to);
 	close(fd);
 }
 #endif