fix: check for 'patched_versions' key

[EslamHiko]

This is a PR to solve this issue : #154

[sbs2001]

Thank you for the PR! , the code will fix the key error but that's not enough . We need to make some changes to the rust-sec importer to adopt to their changed schema. The files with absence of patched_versions field have adopted a new way to denote patched versions, we have to collect that data. So simply checking for the patched_versions field wont fix the problem completely.

[EslamHiko]

@sbs2001 this is the new Schema

{'advisory': 
{
'id': 'RUSTSEC-2019-0016',
 'package': 'chttp',
 'date': '2019-09-01', 
'title': 'Use-after-free in buffer conversion implementation',
'description': 'The From<Buffer> implementation for Vec<u8> was not properly implemented,\nreturning a vector backed by freed memory. This could lead to memory corruption\nor be exploited to cause undefined behavior.\n \nA fix was published in version 0.1.3.\n',
'url': 'https://github.com/sagebind/isahc/issues/2',
'keywords': ['memory-management', 'memory-corruption']
},
 'versions': {'patched': ['>= 0.1.3'], 'unaffected': ['< 0.1.1']}
}

I've updated my PR to work with the new schema:

unaffected_version_range_list = advisory['versions'].get(
            'unaffected', [])
patched_version_range_list = advisory['versions']['patched']

[sbs2001]

LGMT ! @EslamHiko thank you ++ .
@haikoschol please review and merge.

[haikoschol]

@EslamHiko @sbs2001 thank you!