fcgi-client: merge 5 fifo_buffer_consume() calls
fcgi-client: merge 5 fifo_buffer_consume() calls

Since fifo_buffer_consume() does not invalidate existing data, we can
safely call this function before reading the packet header.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy