BUG/MINOR: http: ensure that msg->err_pos is always relative to buf->p

BUG/MINOR: http: ensure that msg->err_pos is always relative to buf->p

Since the beginning of buffer&msg changes, the error position (err_pos)
had not completely been converted and some offsets still appear wrong.
Now we ensure that everywhere msg->err_pos is relative to buf->p and
we always report buf->i bytes starting at buf->p in all error captures,
which ensures that err_pos is there.

This is not exactly a bug and is specific to latest changes so no backport
is needed.