Investigate SPHINCS+ address sanitizer errors on emulated armhf

[jschanck]

One of the CI runs for #418 observed address sanitizer errors for SPHINCS+ on emulated armhf.

Relevant part of the log:

mkdir -p /home/runner/work/PQClean/PQClean/testcases/test_functest_sanitizers__3QjBI6sH/bin
ccache gcc -O3 -Wall -Wextra -Wpedantic -Wvla -Werror -std=c99 -Wundef -Wshadow -Wcast-align -Wpointer-arith -Wmissing-prototypes -fstrict-aliasing -fno-common -pipe -I../common -g -fsanitize=address,undefined -fno-sanitize-recover=undefined -Wno-unused-command-line-argument -latomic -DNTESTS=1 -DPQCLEAN_NAMESPACE=PQCLEAN_SPHINCSSHAKE256192SSIMPLE_CLEAN -I/home/runner/work/PQClean/PQClean/testcases/test_functest_sanitizers__3QjBI6sH/crypto_bla/scheme/impl crypto_sign/functest.c ../common/aes.c ../common/sha2.c ../common/fips202.c ../common/nistseedexpander.c ../common/sp800-185.c ../common/randombytes.c -o /home/runner/work/PQClean/PQClean/testcases/test_functest_sanitizers__3QjBI6sH/bin/functest_sphincs-shake256-192s-simple_clean -L/home/runner/work/PQClean/PQClean/testcases/test_functest_sanitizers__3QjBI6sH/crypto_bla/scheme/impl -lsphincs-shake256-192s-simple_clean

. > /home/runner/work/PQClean/PQClean/testcases/test_functest_sanitizers__3QjBI6sH/bin/functest_sphincs-shake256-192s-simple_clean
==1642==AddressSanitizer CHECK failed: ../../../../src/libsanitizer/asan/asan_poisoning.cpp:36 "((AddrIsInMem(addr))) != (0)" (0x0, 0x0)
=================================================================
==1642==AddressSanitizer CHECK failed: ../../../../src/libsanitizer/asan/asan_descriptions.cpp:80 "((0 && "Address is not in memory and not in shadow?")) != (0)" (0x0, 0x0)

=========================== short test summary info ============================
FAILED test_functest.py::test_functest_sanitizers[clean implementation of sphincs-shake256-192s-simple]
=========== 1 failed, 39 passed, 4801 skipped in 1000.83s (0:16:40) ============

[thomwiggers]

Is this just a bug in asan?...