shared/gatt-server: Fix handle error on execute write

If there is an error during execute write we should drop all
outstanding prep_write data. This patch fix that.
diff --git a/src/shared/gatt-server.c b/src/shared/gatt-server.c
index ba668e3..c41273a 100644
--- a/src/shared/gatt-server.c
+++ b/src/shared/gatt-server.c
@@ -1204,6 +1204,9 @@
 	err = BT_ATT_ERROR_UNLIKELY;
 
 error:
+	queue_remove_all(server->prep_queue, NULL, NULL,
+						prep_write_data_destroy);
+
 	bt_att_send_error_rsp(server->att, BT_ATT_OP_EXEC_WRITE_REQ,
 								ehandle, err);
 }
@@ -1248,6 +1251,8 @@
 	return;
 
 error:
+	queue_remove_all(server->prep_queue, NULL, NULL,
+						prep_write_data_destroy);
 	bt_att_send_error_rsp(server->att, opcode, 0, ecode);
 }