um: fix races between line_open() and line_config()

Pull parse_chan_pair() call into setup_one_line(), under the mutex.
We really don't want open() to succeed before parse_chan_pair() had
been done (or after it has failed, BTW).  We also want "remove con<n>"
to free irqs, etc., same as "config con<n>=none".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/drivers/chan_kern.c b/arch/um/drivers/chan_kern.c
index 420e2c8..3a95498 100644
--- a/arch/um/drivers/chan_kern.c
+++ b/arch/um/drivers/chan_kern.c
@@ -551,6 +551,9 @@
 		INIT_LIST_HEAD(chans);
 	}
 
+	if (!str)
+		return 0;
+
 	out = strchr(str, ',');
 	if (out != NULL) {
 		in = str;