Use strlcpy when copying node IDs into measured_bw_line_t

We were using strncpy before, which isn't our style for stuff like
this.

This isn't a bug, though: before calling strncpy, we were checking
that strlen(src) was indeed == HEX_DIGEST_LEN, which is less than
sizeof(dst), so there was no way we could fail to NUL-terminate.
Still, strncpy(a,b,sizeof(a)) is an idiom that we ought to squash
everyplace.

Fixes CID #427.

src/or/dirserv.c

@@ -2415,7 +2415,7 @@ measured_bw_line_parse(measured_bw_line_t *out, const char *orig_line)
         tor_free(line);
         return -1;
       }
-      strncpy(out->node_hex, cp, sizeof(out->node_hex));
+      strlcpy(out->node_hex, cp, sizeof(out->node_hex));
       got_node_id=1;
     }
   } while ((cp = tor_strtok_r(NULL, " \t", &strtok_state)));