Don't check for null after calling COFFObjectFile::toSec.

It can only return null if passed a corrupted reference with a null Ref.p.
Checking for null is then an issue for asserts to check for internal
consistency, not control flow to check for invalid input.

I didn't add an assert(sec != nullptr) because toSec itself has a far more
complete assert.

llvm-svn: 219235