make `DLLSafeCallback` safer #158

Job	Run time
build-windows (msvc-oak-release)	1m 37s
clang-format	1m 43s
build-windows (clang-oak-release)	3m 18s
build-windows (clang-willow-release)	2m 53s
build-ubuntu (clang-cross-willow-release, clang-cross)	3m 17s
spelling	4s
clang-tidy (clang-oak-release)	11m 32s
build-ubuntu (mingw-oak-release, mingw)	1m 59s
build-ubuntu (llvm-mingw-willow-release, llvm-mingw)	1m 51s
build-ubuntu (llvm-mingw-oak-release, llvm-mingw)	1m 47s
build-ubuntu (clang-cross-oak-release, clang-cross)	3m 10s
build-ubuntu (mingw-willow-release, mingw)	2m 9s
build-windows (msvc-willow-release)	1m 28s
clang-tidy (clang-willow-release)	11m 39s
	48m 27s

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