trivial: Fix compilation issue with GCC 4.5

For some reason, GCC throws an error cause the
"command" var might be not initialized.
So we initialize it now.

src/pk-transaction.c

@@ -687,7 +687,7 @@ pk_transaction_process_script (PkTransaction *transaction, const gchar *filename
 	GFile *file = NULL;
 	GFileInfo *info = NULL;
 	guint file_uid;
-	gchar *command;
+	gchar *command = NULL;
 	gint exit_status = 0;
 	gboolean ret;
 	GError *error = NULL;