input/csv: Accept absence of last end-of-line termination sequence

input/csv: Accept absence of last end-of-line termination sequence

On the Windows platform it appears to be popular to _not_ terminate the
very last line in a text file. Which results in an unmet constraint in
the CSV input module and an internal exception in PulseView which aborts
program execution.

Cope with the absence of the text line termination sequence at the very
end of the input stream. Keep all other checks in place, such that only
completely received text lines get processed.

This fixes bug #635.