Update to 2.5.3 which fixes an integer overflow:

http://blog.bro.org/2018/02/bro-253-released-security-update.html

Note that a CVE has not been assigned yet.

Reviewed by: matthew (mentor)
Approved by: matthew (mentor)
MFH: 2018Q1
Differential Revision: https://reviews.freebsd.org/D14444