fix is_ordinal IndexError

[doomedraven]

 Traceback (most recent call last):
   File "/opt/CAPE/utils/../lib/cuckoo/common/cape_utils.py", line 223, in flare_capa_details
     capabilities, counts = capa.main.find_capabilities(rules, extractor, disable_progress=True)
   File "/usr/local/lib/python2.7/dist-packages/capa/main.py", line 116, in find_capabilities
     function_matches, bb_matches, feature_count = find_function_capabilities(ruleset, extractor, f)
   File "/usr/local/lib/python2.7/dist-packages/capa/main.py", line 68, in find_function_capabilities
     for feature, va in extractor.extract_insn_features(f, bb, insn):
   File "/usr/local/lib/python2.7/dist-packages/capa/features/extractors/viv/__init__.py", line 84, in extract_insn_features
     for feature, va in capa.features.extractors.viv.insn.extract_features(f, bb, insn):
   File "/usr/local/lib/python2.7/dist-packages/capa/features/extractors/viv/insn.py", line 599, in extract_features
     for feature, va in insn_handler(f, bb, insn):
   File "/usr/local/lib/python2.7/dist-packages/capa/features/extractors/viv/insn.py", line 93, in extract_insn_api_features
     for name in capa.features.extractors.helpers.generate_symbols(dll, symbol):
   File "/usr/local/lib/python2.7/dist-packages/capa/features/extractors/helpers.py", line 61, in generate_symbols
     if not is_ordinal(symbol):
   File "/usr/local/lib/python2.7/dist-packages/capa/features/extractors/helpers.py", line 45, in is_ordinal
     return symbol[0] == "#"
 IndexError: string index out of range

[mr-tz]

ah, good catch, what's the API feature being parsed here?

[mr-tz]

likely memdump or obfuscated sample
vivisect identifies imports {268484864: ('', ''), 268484608: ('', ''), 268484740: ('', ''), ...