cstool: fix memleak to prevent ASAN from complaining (#1222)

diff --git a/cstool/cstool.c b/cstool/cstool.c
index 65ad945..303c553 100644
--- a/cstool/cstool.c
+++ b/cstool/cstool.c
@@ -395,6 +395,7 @@
 	}
 
 	cs_close(&handle);
+	free(assembly);
 
 	return 0;
 }