When we have a command returned by zfs_nextboot() that is longer

than command in the loader.conf, the latter needs to be nul terminated,
otherwise garbage trailer left from zfs_nextboot() will be passed to
parse_cmd() together with loader.conf command.

While here, reset cmd to empty string if read() returns error.

Reviewed by:	tsoome

stand/i386/zfsboot/zfsboot.c

@@ -248,7 +248,12 @@ main(void)
 		fd = open(PATH_DOTCONFIG, O_RDONLY);
 
 	if (fd != -1) {
-		read(fd, cmd, sizeof (cmd));
+		ssize_t cmdlen;
+
+		if ((cmdlen = read(fd, cmd, sizeof(cmd))) > 0)
+			cmd[cmdlen] = '\0';
+		else
+			*cmd = '\0';
 		close(fd);
 	}