[MC] Fix an assert in MCAssembler::writeSectionData to be aware of errors

If MCContext has an error, MCAssembler::layout may stop early
and some MCFragment's may not finalize.

In the Linux kernel, arch/x86/lib/memcpy_64.S could trigger the assert before
"x86_64: Change .weak to SYM_FUNC_START_WEAK for arch/x86/lib/mem*_64.S"