fix some ELF parsing vulnerabilities #255
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.