subs/wu: Avoid "may be used uninitialized" warnings

subs/wu: Avoid "may be used uninitialized" warnings

typed_wu() either sends a telegram to a single recipient @to, or an
announcement.  It initializes @np to getnatp(to) only when sending a
telegram.  It also uses it only then.  Clang is cool with this, but
GCC gets confused and warns "‘np’ may be used uninitialized".  Ron
Koenderink reports Visual Studio 2019 even flags it as an error.

Time to clean this up.  Initialize @np to NULL when sending an
announcement.  Fuse conditionals while there.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>