Fix missing NUL-terminator in grep

Fix missing NUL-terminator in grep

Currently, grep read()s into a buffer and then uses the buffer as a
string. Since there's no NUL-terminator, this can cause it to falsely
identify line breaks and matches from leftover data on earlier lines
and, if a line fills up the entire buffer, to read past the end of the
buffer.

Fix this by NUL-terminating any data returned by read().

Thanks to Keiichi Watanabe for the report.