Fix out-of-bounds read in objdump reloc section handling

[sumleo]

Summary

• BinaryReaderObjdump::OnRelocCount ignored the error returned by BinaryReaderObjdumpBase::OnRelocCount when the section_index was invalid, causing it to proceed to GetSectionName → GetSectionStart with BinarySection::Invalid (~0), resulting in an out-of-bounds read on the stack-allocated section_starts_ array of size kBinarySectionCount (14).
• Propagate the error via CHECK_RESULT so the out-of-bounds access is never reached.
• Add a regression test with a crafted wasm binary containing a reloc custom section that references a non-existent section index.

Details

In binary-reader-objdump.cc, BinaryReaderObjdump::OnRelocCount calls the base class method but discards its return value:

Result BinaryReaderObjdump::OnRelocCount(Index count, Index section_index) {
  BinaryReaderObjdumpBase::OnRelocCount(count, section_index); // return value ignored
  PrintDetails("  - relocations for section: %d (%s) [%d]\n",
               section_index,
               GetSectionName(section_index),  // OOB read here
               count);
  return Result::Ok;
}

The base class returns Result::Error for out-of-range indices and sets reloc_section_ = BinarySection::Invalid. GetSectionName then calls GetSectionStart(BinarySection::Invalid) which indexes section_starts_[~0] — far past the 14-element array.

Test plan

• New test BinaryReaderObjdump.RelocInvalidSectionIndex exercises the fix with a crafted wasm binary
• All existing unit tests pass (128 tests)