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 <[email protected]>
Reviewed-by: Serapheim Dimitropoulos <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: George Wilson <[email protected]>
Closes #13781

Author: grwilson

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));