-Werror=class-memaccess (trunk towards GCC 9)

..."‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type";
instead adding default member initializers to the members of WW8Fib and WW8Dop,
where all ctors called memset.

Unfortunately, C++17 doesn't yet allow default initializers for bit-field
members, so those still need to be covered by per-ctor initializer lists for
now.

Change-Id: I0f4b5130d92abed2f808084813ff1089d390c809
Reviewed-on: https://gerrit.libreoffice.org/65376
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>