GenBank: Don't die parsing files with too short feature qualifiers

I just stumbled over an input file that didn't specify a locus for some
misc_feature in production. This caused the GenBank parser to die with
an IndexError. As that's not the greatest error message to return to
library users, just handle and ignore invalid feature qualifiers.

Signed-off-by: Kai Blin <kai.blin@biotech.uni-tuebingen.de>