Reduce number of `malloc/free` call in `XMSS/external` (#1724) #1561

Job	Run time
Check code formatting	1m 8s
Check upstream code is properly integrated	1m 13s
Check that code passes a basic build before starting heavier tests	1m 4s
linux_cross_compile (windows-binaries, -DCMAKE_TOOLCHAIN_FILE=../.CMake/toolchain_windows-amd64.c...	1m 26s
linux_intel (alpine, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WARNINGS=ON -DOQS_USE_O...	9m 6s
linux_cross_compile (windows-dll, -DCMAKE_TOOLCHAIN_FILE=../.CMake/toolchain_windows-amd64.cmake ...	1m 26s
linux_arm_emulated (armhf, armhf, -DOQS_ENABLE_SIG_SPHINCS=OFF -DOQS_USE_OPENSSL=OFF -DOQS_OPT_TA...	31m 46s
linux_intel (alpine-no-stfl-key-sig-gen, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WAR...	5m 22s
linux_arm_emulated (armhf-no-stfl-key-sig-gen, armhf, -DOQS_ENABLE_SIG_SPHINCS=OFF -DOQS_USE_OPEN...	14m 40s
linux_intel (alpine-openssl-all, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WARNINGS=ON...	17m 13s
linux_intel (alpine-noopenssl, openquantumsafe/ci-alpine-amd64:latest, -DOQS_STRICT_WARNINGS=ON -...	11m 4s
linux_intel (focal-nistr4-openssl, openquantumsafe/ci-ubuntu-focal-x86_64:latest, -DOQS_STRICT_WA...	3m 42s
linux_intel (jammy-std-openssl3, openquantumsafe/ci-ubuntu-jammy:latest, -DOQS_STRICT_WARNINGS=ON...	3m 32s
linux_intel (address-sanitizer, openquantumsafe/ci-ubuntu-focal-x86_64:latest, -DCMAKE_C_COMPILER...	12m 20s
linux_intel (address-sanitizer-no-stfl-key-sig-gen, openquantumsafe/ci-ubuntu-focal-x86_64:latest...	8m 30s
	2h 3m 32s

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