goto-instrument behaviour when not given an output file name

[polgreen]

goto-instrument should check whether it has been given an output file name before getting into expensive analysis steps, not afterwards.

Currently, running the command
goto-instrument --full-slicer input.binary will result in goto-instrument running the full slicer, before realising that it hasn't got an output file name, which causes it to output the help information.

It would be better to check whether the output file name has been given before running the analysis, and to provide a better error message, e.g., incorrect command given, to view help run goto-instrument --help because:

• running the analysis could be time consuming, e.g., the full slicer can take several days to complete, which is wasted if there is no output file
• it's not entirely obvious that outputting the help file means something was wrong with the command given
• the help file is so large that it makes it hard to immediately see that any analysis has run before hand

[martin-cs]

+1 agree; all command line parsing should be done first, in goto_instrument_parse_options.