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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219235 91177308-0d34-0410-b5e6-96231b3b80d8