MemorySanitizer doesn't wrap explicit_bzero

[mkj]

The attached oss-fuzz case for Dropbear SSH reports uninitialised memory. It appears that explicit_bzero() isn't handled correctly by msan, replacing it with plain memset() in dbhelpers.c m_burn() resolves the sanitizer error.

The call in the testcase is coming from ecc_ansi_x963_export() -> zeromem() -> m_burn() -> explicit_bzero().

14630_crash_info_dropbear_Use-of-uninitialized-value.txt

Uninitialized value was created by an allocation of 'buf' in the stack frame of function 'ecc_ansi_x963_export'

Base64 testcase: AgAAAfEAAACFBAHqNMmoriKZ+LgWqFtiHuKXnW+pQTQEZJMbVZogafvfwq/5uWgu7t2uhzzq/WOINVoScyhoWmZgemjurp6R01di+gGRkC0EirjG5EtweiiNIua4hMdoQdR36go+Sr7sDFdHgh0XsNwPIHMCVy8n90AdCH6l/+rSBZDrQf4OUN2gYcnsAQ==

[salzmdan]

FYI I came across the same issue. I have solved that by
CZ-NIC/knot@564f616