lib: assert(0) still needs a return

assert(0) is not guaranteed to not return since assert() in general can
be optimised out when building without debug / with optimisation.  This
breaks the build in clang, which warns/errors about the missing return.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/command.c b/lib/command.c
index c70391d..8ae27de 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -2018,6 +2018,7 @@
 
       default:
         assert(0);
+        return NULL;
     }
 }