[LTO] Handle null GV in Symbol object

Similar to other Symbol methods, have Symbol::getComdat handle
a null GV gracefully.

Fixes PR30326.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281134 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed