Clear up another clangalyzer issue

"The NULL pointer warnings on the return value of
tor_addr_to_in6_addr32() are incorrect. But clang can't work this
out itself due to limited analysis depth. To teach the analyser that
the return value is safe to dereference, I applied tor_assert to the
return value."

Patch from teor.  Part of 13157.
Clean up a clangalyzer warning in directory_remove_invalid

"At this point in the code, msg has been set to a string
constant. But the tor code checks that msg is not NULL, and the
redundant NULL check confuses the analyser[...] To avoid this
spurious warning, the patch initialises msg to NULL."

Patch from teor. another part of 13157.