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.

grep.c

@@ -14,8 +14,9 @@ grep(char *pattern, int fd)
   char *p, *q;
 
   m = 0;
-  while((n = read(fd, buf+m, sizeof(buf)-m)) > 0){
+  while((n = read(fd, buf+m, sizeof(buf)-m-1)) > 0){
     m += n;
+    buf[m] = '\0';
     p = buf;
     while((q = strchr(p, '\n')) != 0){
       *q = 0;