lib, staticd: return values even after an assert

lib, staticd: return values even after an assert

When compiling with -fsanitize=thread.  I started getting this error:

staticd/static_zebra.c: In function ‘static_zebra_nht_get_prefix’:
staticd/static_zebra.c:316:1: error: control reaches end of non-void function [-Werror=return-type]
  316 | }
      | ^

Just to make future efforts still work, let's just make the compiler happy.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>