Fix for corrupted Nortek files

[jmcvey3]

If a datafile is filled with bad bytes, the reader will get stuck in one of two while loops.

[akeeste]

@jmcvey3 Using the sample file I sent you, this fix gets past the previous sticking point in #370 (_NortekReader initialization), but now gets stuck at _NortekReader.readfile(). Is there another check you can add that will break the while loops in _NortekReader.readfile() and/or _NortekReader.findnext()?

[jmcvey3]

@jmcvey3 Using the sample file I sent you, this fix gets past the previous sticking point in #370 (_NortekReader initialization), but now gets stuck at _NortekReader.readfile(). Is there another check you can add that will break the while loops in _NortekReader.readfile() and/or _NortekReader.findnext()?

How long did you let it run for, and what is listed in the .dolfyn.log? It took a good 5 or 10 minutes for dolfyn to cycle through every byte of the file to try and find valid data.

[akeeste]

@jmcvey3 got it. I'm not sure what was slowing down my test last week, but I tested again this morning and it finished in about 10 minutes. The log file looks as expected too, so this PR seems good to go.