Fix uninitialized variables. Found by ubsan.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@178913 91177308-0d34-0410-b5e6-96231b3b80d8
Correctly pass ownership of MemoryBuffers.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@178914 91177308-0d34-0410-b5e6-96231b3b80d8