Set EB value before checking for user agent string length

Summary: Set EB value before checking for user agent string length

Test Plan: execute make -j4 src/test/test_bitcoin && src/test/test_bitcoin -t net_tests

Reviewers: freetrader, deadalnix, kyuupichan, awemany, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D293