Avoid negative overflow when the text output driver tries to print

Avoid negative overflow when the text output driver tries to print
random binary data.

libpspp/u8-line.c (u8_line_find_pos): Deal properly with the case
where the target is not found in the search string.

libpspp/u8-line.c (u8_line_reserve): Add some assertions to prevent
negative overflow into lower function calls.

Reported by: Andrea Fioraldi

Fixes bug: #58591