GPU (Linux): fix memleaks #793

Job	Run time
Linux-amd64	3m 13s
FreeBSD-amd64	3m 5s
Linux-aarch64	10m 45s
FreeBSD-aarch64	15m 57s
Linux build test (gcc, g++, ON)	48s
Linux-old-amd64	3m 31s
macOS-universal	11m 43s
Linux build test (gcc, g++, OFF)	55s
Linux build test (clang, clang++, ON)	1m 2s
spellcheck	11s
Windows-amd64	2m 30s
Linux build test (clang, clang++, OFF)	1m 0s
Windows-i686	6m 54s
Linux build test (musl-gcc, musl-g++, OFF)	48s
Release	0s
	1h 2m 22s

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