BUG/MEDIUM: mux-fcgi: fix uint16_t overflow in drl += drp

BUG/MEDIUM: mux-fcgi: fix uint16_t overflow in drl += drp

The FCGI demux record length field (drl) is uint16_t. In the
ignore_record path, the expression "fconn->drl += fconn->drp" overflows
to 0 when contentLength=65535 and paddingLength>=1. This causes the
state machine to consider the record complete without consuming any
buffer data. The remaining buffer contents are then parsed as new FCGI
record headers.

The same drl+=drp pattern at lines 2382/2418/2475 is not affected
because drl is guaranteed to be 0 at those points (all content bytes
are consumed before reaching end_transfer).

Widen drl from uint16_t to uint32_t so that the addition of drp
(uint8_t, max 255) cannot overflow.

Reported-by: Tristan (@TristanInSec)