Fixes #639: Debugger Failure when encountering "=" prefixed output

**This is a hack**. for some unknown reason, GDB is outputting lines which are clearly stdout from the debugged program, but are not `@` prefixed. This means that lines that happen to start with mi2 control characters are being passed into mi2 parsing functions that themselves do *very little* validation before `substring`ing everywhere.

That being said, it works, and it should protect against a whole class of invalid parsing (as described in #639).