bugfix: fix prospector linter to not fail on unset line number.

[arthepsy]

While using prospector linter, I encountered strange behavior. It gave this output:

Linting with prospector failed.
TypeError: Cannot read property 'substring' of undefined

Debugged the code and found out that it was crashing on this PEP8 message:

E101: indentation contains mixed spaces and tabs

Turns out, this particular lint message (and probably others aswell), doesn't contain line number, therefore, in src/client/linters/prospector.ts, this line

let sourceLine = txtDocumentLines[msg.location.line - 1];

got assigned undefined and the next line

let sourceStart = sourceLine.substring(msg.location.character);

crashed with error mentioned above.

This pull request fixes the issue.