grep: NUL terminate input from a file

Internally "git grep" runs regexec(3) that expects its input string
to be NUL terminated.  When searching inside blob data, read_sha1_file()
automatically gives such a buffer, but builtin-grep.c forgot to put
the NUL at the end, even though it allocated enough space for it.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -207,6 +207,7 @@ static int grep_file(struct grep_opt *opt, const char *filename)
 		return 0;
 	}
 	close(i);
+	data[sz] = 0;
 	if (opt->relative && opt->prefix_length)
 		filename = quote_path_relative(filename, -1, &buf, opt->prefix);
 	i = grep_buffer(opt, filename, data, sz);