src/gatt-client: Fix iteration through options

Fix infinite loop when trying to read/write a characteristic or
a descriptor and the ReadValue/WriteValue call has at least one
option.
diff --git a/src/gatt-client.c b/src/gatt-client.c
index 0cbacca..6fc0d19 100644
--- a/src/gatt-client.c
+++ b/src/gatt-client.c
@@ -401,6 +401,8 @@
 				return -EINVAL;
 			dbus_message_iter_get_basic(&value, offset);
 		}
+
+		dbus_message_iter_next(&dict);
 	}
 
 	return 0;