Fix a build failure in the new analyzer module that was reported with
clang 9.0.1 (only):
ports/lang/gcc10-devel/work/gcc-10-20200126/gcc/analyzer/engine.cc:2971:13: error:
reinterpret_cast from 'nullptr_t' to 'function *' is not allowed
v.m_fun = reinterpret_cast<function *> (NULL);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~PR: 243681
Reported by: Philippe Michel <philippe.michel7@free.fr>