[PATCH v2] Silence most -Wzero-as-null-pointer-constant diagnostics

[Joseph Myers]

Have the changes to zic.c gone upstream?  See SHARED-FILES and (older) 
https://sourceware.org/glibc/wiki/SharedSourceFiles - we shouldn't 
generally have local changes to the files from tzcode.

-- 
Joseph S. Myers
josmyers@redhat.com