stdio/file: fix write_buffer() #1279

Job	Run time
call-ci / build (armv7a7-imx6ull-evk)	1m 8s
call-ci / build (armv7a9-zynq7000-qemu)	1m 22s
call-ci / build (armv7a9-zynq7000-zedboard)	1m 3s
call-ci / build (armv7a9-zynq7000-zturn)	58s
call-ci / build (armv7m4-stm32l4x6-nucleo)	1m 2s
call-ci / build (armv7m7-imxrt105x-evk)	1m 9s
call-ci / build (armv7m7-imxrt106x-evk)	1m 8s
call-ci / build (armv7m7-imxrt117x-evk)	1m 11s
call-ci / build (armv8r52-mps3an536-qemu)	1m 11s
call-ci / build (armv8m33-mcxn94x-frdm)	1m 8s
call-ci / build (host-generic-pc)	55s
call-ci / build (ia32-generic-pc)	1m 0s
call-ci / build (ia32-generic-qemu)	1m 3s
call-ci / build (riscv64-generic-qemu)	51s
call-ci / build (riscv64-generic-spike)	52s
call-ci / build (sparcv8leon-generic-qemu)	50s
call-ci / build (sparcv8leon-gr712rc-board)	54s
call-ci / build (sparcv8leon-gr716-mimas)	53s
call-ci / build (sparcv8leon-gr716-mini)	52s
call-ci / test EMU (armv7a9-zynq7000-qemu)	8m 13s
call-ci / test EMU (host-generic-pc)	55s
call-ci / test HW (armv7a7-imx6ull-evk)	6m 20s
call-ci / test EMU (ia32-generic-qemu)	6m 45s
call-ci / test HW (armv7m7-imxrt106x-evk)	2m 40s
call-ci / test EMU (riscv64-generic-qemu)	5m 42s
call-ci / test HW (armv7m7-imxrt117x-evk)	2m 49s
call-ci / test EMU (sparcv8leon-generic-qemu)	2m 50s
call-ci / test HW (armv7a9-zynq7000-zedboard)	6m 41s
call-ci / test HW (armv7m4-stm32l4x6-nucleo)	11m 36s
call-ci / tests-summary	23s
call-ci / send-notification	0s
	1h 14m 24s

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