[COFF] Fix error handling in ResourceSectionRef

[COFF] Fix error handling in ResourceSectionRef

Previously, the expression (Reader.readFoo()) was expanded twice,
triggering asserts as one of the Error types ends up not checked
(and as it was expanded twice, the method would end up called twice
if it failed first).

Differential Revision: https://reviews.llvm.org/D66817

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370309 91177308-0d34-0410-b5e6-96231b3b80d8