db.c: tmsi may be empty so put it into '' to make that obvoius (Andreas Eversberg)

during attachment, the "tmsi" field of the subscriber may be an empty
string, so the string must be quoted in the sql request, or the sql
request fails due to incorrect syntax.

also i added "extension" field to debug output.
diff --git a/src/db.c b/src/db.c
index 4abd636..600699a 100644
--- a/src/db.c
+++ b/src/db.c
@@ -261,8 +261,8 @@
 
 	subscr->lac = dbi_result_get_uint(result, "lac");
 	subscr->authorized = dbi_result_get_uint(result, "authorized");
-	printf("DB: Found Subscriber: ID %llu, IMSI %s, NAME '%s', TMSI %s, LAC %hu, AUTH %u\n",
-		subscr->id, subscr->imsi, subscr->name, subscr->tmsi,
+	printf("DB: Found Subscriber: ID %llu, IMSI %s, NAME '%s', TMSI %s, EXTEN '%s', LAC %hu, AUTH %u\n",
+		subscr->id, subscr->imsi, subscr->name, subscr->tmsi, subscr->extension,
 		subscr->lac, subscr->authorized);
 	dbi_result_free(result);
 	return subscr;
@@ -273,7 +273,7 @@
 	result = dbi_conn_queryf(conn,
 		"UPDATE Subscriber "
 		"SET updated = datetime('now'), "
-		"tmsi = %s, "
+		"tmsi = '%s', "
 		"lac = %i, "
 		"authorized = %i "
 		"WHERE imsi = %s ",