Implement the recommendations described in issue #255 by @zzazzdzz:

- Check bounds when reading ELF program header sections.

- Skip reading ELF section headers if the string table pointer is NULL.

- Increase the buffer size for dissassembled instructions in the
  dissassembly view and pass the buffer size to the disArm() and
  disThumb() functions so that rudimentary bounds checking can be done.

Also add the constants WORK_RAM_SIZE and ROM_SIZE to reduce incidence of
magic numbers and make the code a bit cleaner.