CLEANUP: http: fix a build warning introduced by a recent fix

CLEANUP: http: fix a build warning introduced by a recent fix

Cyril reported that recent commit 320ec2a ("BUG/MEDIUM: chunks: always
reject negative-length chunks") introduced a build warning because gcc
cannot guess that we can't fall into the case where the auth_method
chunk is not initialized.

This patch addresses it, though for the long term it would be best
if chunk_initlen() would always initialize the result.

This fix must be backported to 1.6 and 1.5 where the aforementionned
fix was already backported.