Fix memory leak in verifier.
Fix memory leak in verifier.

We create std::ostringstream objects for failure messages but it appears we did
not delete them upon destruction. This CL fixes this.

Change-Id: If95c6f1b154db9c1d49ade4c1384ef6015663419

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst