ipc4: fix an "unused variable" warning, found by a fuzzer #20499

Job	Run time
LP64-WIP	4m 6s
build-windows (imx8 imx8x imx8m imx8ulp)	8m 10s
build-windows (ptl)	6m 33s
manifest-check	56s
build-windows (tgl tgl-h)	7m 17s
build-windows (-d, lnl)	6m 29s
build-windows (mtl lnl)	7m 36s
build-linux (mnfst, --all)	14m 38s
build-linux (-d, mnfst, lnl)	4m 33s
build-linux (mnfst, mtl lnl)	5m 59s
build-linux (zmain, ptl)	4m 32s
build-linux (mnfst, ptl)	5m 20s
build-linux (mnfst, tgl tgl-h)	5m 37s
build-linux (zmain, tgl tgl-h)	5m 27s
build-linux (mnfst, imx8 imx8x imx8m imx8ulp)	6m 51s
build-linux (zmain, mtl lnl)	6m 33s
build-linux (zmain, imx8 imx8x imx8m imx8ulp)	6m 26s
compare-linux-win	35s
	1h 47m 38s

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