shared/btp: Allow to call btp_send without valid btp

This allows to unconditionally call btp_send from code that generates
events eg when mapping those from D-Bus events.
diff --git a/src/shared/btp.c b/src/shared/btp.c
index 7762267..93df37a 100644
--- a/src/shared/btp.c
+++ b/src/shared/btp.c
@@ -294,6 +294,9 @@
 	struct pending_message *msg;
 	size_t len;
 
+	if (!btp)
+		return false;
+
 	len = sizeof(*hdr) + length;
 	hdr = l_malloc(len);
 	if (!hdr)