[X86][Disassembler] Fix a bug when disassembling an empty string

readPrefixes() assumes insn->bytes is non-empty. The code path is not
exercised in llvm-mc because llvm-mc does not feed empty input to
MCDisassembler::getInstruction().

This bug is uncovered by a5994c78.
An empty string did not crash before because the deleted regionReader()
allowed UINT64_C(-1) as insn->readerCursor.

  Bytes.size() <= Address -> R->Base
  0 <= UINT64_C(-1) - UINT32_C(-1)