Don't use eval() in read_bss_sections

[kanzure]

kanzure@deathfortress:~/code/pokecrystal$ python extras/pokemontools/gbz80disasm.py 0x45f
Traceback (most recent call last):
  File "extras/pokemontools/gbz80disasm.py", line 934, in <module>
    disasm.initialize()
  File "extras/pokemontools/gbz80disasm.py", line 619, in initialize
    self.wram.initialize()
  File "/home/kanzure/code/pokecrystal/extras/pokemontools/wram.py", line 106, in initialize
    self.setup_wram_sections()
  File "/home/kanzure/code/pokecrystal/extras/pokemontools/wram.py", line 128, in setup_wram_sections
    self.wram_sections = self.read_wram_sections()
  File "/home/kanzure/code/pokecrystal/extras/pokemontools/wram.py", line 121, in read_wram_sections
    wram_sections = read_bss_sections(wram_content)
  File "/home/kanzure/code/pokecrystal/extras/pokemontools/wram.py", line 48, in read_bss_sections
    length = eval(line[3:line.find(';')].replace('$','0x'))
  File "<string>", line 1, in <module>
NameError: name 'OBJECT_LENGTH' is not defined

Using eval is wrong many times over. Let's use something else.