fixed srec parsing issues for files with additional data at the end

[jstucke]

There are frequent unpacking errors when trying to unpack srec files that were carved by binwalk and have additional data at the end. This PR should ensure that the correct file end is used when parsing those records.

I also changed the "bad" test files from syntax errors to "valid errors" (as far as the syntax is concerned): checksum mismatch and wrong length).