Avoid out-of-bounds section_starts_ read on invalid reloc index

[aizu-m]

UBSan, wasm-objdump -x on a crafted module:

binary-reader-objdump.cc:69:12: runtime error: index 18446744073709551615 out of bounds for type 'const Offset[14]'
    #0 BinaryReaderObjdumpBase::GetSectionStart binary-reader-objdump.cc:69
    #1 BinaryReaderObjdump::OnReloc binary-reader-objdump.cc:2176

The input is a reloc.* custom section whose section index points past the last section. The base OnRelocCount spots that, sets reloc_section_ to BinarySection::Invalid (~0) and returns Error to abort the section. The details-mode override dropped that result and returned Ok, so ReadRelocSection carried on into OnReloc, where GetSectionStart(reloc_section_) reads section_starts_[(size_t)-1].

Result is not [[nodiscard]], so the dropped error built clean. Propagating it with CHECK_RESULT rejects the bad index the same way the other reader modes already do.

bad-relocs.txt used a zero relocation count, so it never reached OnReloc; gave it one entry so it covers the read.