Bug/get symbol from list local var use before assignment

[MIvanchev]

Fixes #60. I've extended the unit tests to cover the error. Some things to notice about this pull requests:

• I've consolidated the logic into a single function and removed the new get_symbol_from_list. It seems that it was never intended to be part of the interface so no harm done there.

• get_symbol_list was nullifying the size of the returned symbol. I see no reason for that so now the size is left intact.

• I've refactored the names to make it more clear what the function is doing. It looks for the symbol with the closest starting address <= address where symbols which fully contain the address have priority.

PS: perhaps somebody can do a quick review =)

[MarioVilas]

I believe this no longer applies since the function was rewritten anyway.