isdn: prevent NULL ptr Oops in capi_cmsg2str()

The dereferencing of the private pointer cmsg->m in capi_cmsg2str() may
cause an Oops in case of an error, which is particularly inconvenient
as that function is typically used to format an error message. Add a
NULL pointer check to avoid this.

Impact: error handling improvement
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/capi/capiutil.c

@@ -989,6 +989,8 @@ _cdebbuf *capi_cmsg2str(_cmsg * cmsg)
 {
 	_cdebbuf *cdb;
 
+	if (!cmsg->m)
+		return NULL;	/* no message */
 	cdb = cdebbuf_alloc();
 	if (!cdb)
 		return NULL;