BUG/MINOR: proxy: unsafe initialization of HTTP transaction when switching from TCP...

BUG/MINOR: proxy: unsafe initialization of HTTP transaction when switching from TCP frontend

A switch from a TCP frontend to an HTTP backend initializes the HTTP
transaction. txn->hdr_idx.size is used by hdr_idx_init() but not
necessarily initialized yet here, because the first call to hdr_idx_init()
is in fact placed in http_init_txn(). Moving it before the call is
enough to fix it. We also remove the useless extra confusing call
to hdr_idx_init().

The bug was introduced in 1.5-dev8 with commit ac1932d ("MEDIUM:
tune.http.maxhdr makes it possible to configure the maximum number
of HTTP headers"). No backport to stable is needed.