Nightly-multi-test valgrind fix

[rlm2002]

Description

Increase rand buffer, fix valgrind invalid read size 16

Testing

./configure --srcdir=. --disable-jobserver --enable-option-checking=fatal \
--enable-all --enable-acert --enable-dtls13 --enable-dtls-mtu --enable-dtls-frag-ch \
--enable-dtlscid --enable-quic --with-sys-crypto-policy --enable-intelasm \
--enable-sp-math-all --enable-smallstack --enable-smallstackcache \
CPPFLAGS= "-DNO_WOLFSSL_CIPHER_SUITE_TEST -DWOLFSSL_OLD_PRIME_CHECK -pedantic -Wdeclaration-after-statement -Wnull-dereference -DTEST_LIBWOLFSSL_SOURCES_INCLUSION_SEQUENCE -Werror" \
CFLAGS="-DTEST_ALWAYS_RUN_TO_END -g -fno-omit-frame-pointer" LDFLAGS="-g -fno-omit-frame-pointer -lm"

make

LD_LIBRARY_PATH=./src/.libs valgrind --leak-check=full --show-leak-kinds=all \
--errors-for-leak-kinds=all  --show-reachable=yes  --leak-resolution=high \
--track-fds=yes --track-origins=yes --fullpath-after=  --error-exitcode=10 \
./testsuite/.libs/testsuite.test