db_hlr.c: add db_subscr_exists_by_msisdn()

Check if a subscriber exists without generating an error log entry if
it does not. This is cheaper than db_subscr_get_by_msisdn(), as it
does not fetch the subscriber entry.

subscriber-create-on-demand will use this function to generate
a random unique MSISDN for new subscribers.

Related: OS#2542
Change-Id: Ibfbc408c966197682ba2b12d166ade4bfeb7abc2
diff --git a/src/db.h b/src/db.h
index 8543085..12e74f8 100644
--- a/src/db.h
+++ b/src/db.h
@@ -29,6 +29,7 @@
 	DB_STMT_AUC_3G_DELETE,
 	DB_STMT_SET_LAST_LU_SEEN,
 	DB_STMT_EXISTS_BY_IMSI,
+	DB_STMT_EXISTS_BY_MSISDN,
 	_NUM_DB_STMT
 };
 
@@ -132,6 +133,7 @@
 int db_subscr_update_imei_by_imsi(struct db_context *dbc, const char* imsi, const char *imei);
 
 int db_subscr_exists_by_imsi(struct db_context *dbc, const char *imsi);
+int db_subscr_exists_by_msisdn(struct db_context *dbc, const char *msisdn);
 
 int db_subscr_get_by_imsi(struct db_context *dbc, const char *imsi,
 			  struct hlr_subscriber *subscr);