Crash on malformed input files due to out of range arguments

[jasperla]

An input file such as this, as created and minimized with AFL:

ssdeep,1.0--blocksize:hash:hash,filename
"

crashes ssdeep when provided as input to ssdeep -m $FILE:

% ./inst/bin/ssdeep -m ssdeep.crash
terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::substr: __pos (which is 2) > this->size() (which is 1)
zsh: abort      ./inst/bin/ssdeep -m ssdeep.crash
%