http: fix body tracking corner case

http: fix body tracking corner case

In some cases, observed with inspect limits 0, the body tracking could
get confused. When all chunks were already freed, a new chunk would
be considered to be the start of the body. This would overwrite the
bodies 'content_len_so_far' tracker, instead of adding to it. This in
turn could lead to a assertion abort in the inspection code.

This patch redoes the append code to always add the current lenght. It
cleans up the code to remove redundant logic.

Issue: https://redmine.openinfosecfoundation.org/issues/2078
Reported-By: Jørgen Bøhnsdalen