sys/net/nanocoap: fix `coap_build_reply_header()` #31591

Job	Run time
tools-build (benchmark_udp, dist/tools)	15s
tools-build (bossa-1.8, dist/tools)	19s
tools-build (bossa-1.9, dist/tools)	23s
tools-build (bossa-nrf52, dist/tools)	23s
tools-build (calc_spi_scalers, cpu/kinetis/dist)	16s
tools-build (clk_conf, cpu/stm32/dist)	15s
tools-build (edbg, dist/tools)	18s
tools-build (ethos, dist/tools)	15s
tools-build (fixdep, dist/tools)	15s
tools-build (flatc, dist/tools)	2m 11s
tools-build (lpc2k_pgm, dist/tools)	14s
tools-build (mosquitto_rsmb, dist/tools)	21s
tools-build (riotboot_serial, dist/tools)	15s
tools-build (setsid, dist/tools)	15s
tools-build (uhcpd, dist/tools)	13s
tools-build (zep_dispatch, dist/tools)	16s
tools-build-success	1s
	6m 25s

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