`advanceSubsection()` didn't account for the possibility that a section could have no subsections. Reviewed By: #lld-macho, thakis, BertalanD Differential Revision: https://reviews.llvm.org/D130288
`advanceSubsection()` didn't account for the possibility that a section could have no subsections. Reviewed By: #lld-macho, thakis, BertalanD Differential Revision: https://reviews.llvm.org/D130288