lib, ovsdb, vtep: Add various null pointer checks.

This commit adds various null pointer checks to some files in the `lib`,
`ovsdb`, and `vtep` directories to fix several Coverity defects. These
changes are grouped together as they perform similar checks, returning
early, skipping some action, or logging a warning if a null pointer is
encountered.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: James Raphael Tiovalen <jamestiotio@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

lib/shash.c

@@ -205,6 +205,10 @@ shash_delete(struct shash *sh, struct shash_node *node)
 char *
 shash_steal(struct shash *sh, struct shash_node *node)
 {
+    if (!node) {
+        return NULL;
+    }
+
     char *name = node->name;
 
     hmap_remove(&sh->map, &node->node);