*: add/cleanup initialisers

There were some (inconsequential) warnings about uninitialised use of
variables.  Also, in one case, sub-structs were mixed in initialisation,
which doesn't quite work as intended.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/command.c b/lib/command.c
index 8317789..f20065f 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -1353,7 +1353,7 @@
   enum match_type multiple_match;
   unsigned int multiple_index;
   const char *word;
-  const char *arg;
+  const char *arg = NULL;
   struct cmd_token *word_token;
   enum match_type word_match;