Importing from cachefile can trip assertion

When importing from cachefile, it is possible that the builtin retry
logic will trip an assertion because it also fails to find the pool.
This fix addresses that case and returns the correct error message to
the user.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Wilson <gwilson@delphix.com>
Closes #13781

lib/libzutil/zutil_import.c

@@ -1694,6 +1694,8 @@ zpool_find_import_cached(libpc_handle_t *hdl, importargs_t *iarg)
 			 * caller.
 			 */
 			nvpair_t *pair = nvlist_next_nvpair(nv, NULL);
+			if (pair == NULL)
+				continue;
 			fnvlist_add_nvlist(pools, nvpair_name(pair),
 			    fnvpair_value_nvlist(pair));