client: Add missing duplicated string free

This patch free duplicated strings in read, write attribute callbacks.
diff --git a/client/gatt.c b/client/gatt.c
index d59d1ba..c8267a7 100644
--- a/client/gatt.c
+++ b/client/gatt.c
@@ -1598,6 +1598,7 @@
 
 		bt_shell_prompt_input("gatt", str, authorize_read_response,
 									aad);
+		g_free(str);
 
 		pending_message = dbus_message_ref(msg);
 
@@ -1700,6 +1701,7 @@
 
 		bt_shell_prompt_input("gatt", str, authorize_write_response,
 									aad);
+		g_free(str);
 
 		pending_message = dbus_message_ref(msg);