Fix NULL pointer dereference in spa_open_common()

Calling spa_open() will pass a NULL pointer to spa_open_common()'s
config parameter. Under the right circumstances, we will dereference the
config parameter without doing a NULL check.

Clang's static analyzer found this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14044

Author: ryao

module/zfs/spa.c

@@ -5230,7 +5230,7 @@ spa_open_common(const char *pool, spa_t **spapp, void *tag, nvlist_t *nvpolicy,
 	 * If we've recovered the pool, pass back any information we
 	 * gathered while doing the load.
 	 */
-	if (state == SPA_LOAD_RECOVER) {
+	if (state == SPA_LOAD_RECOVER && config != NULL) {
 		fnvlist_add_nvlist(*config, ZPOOL_CONFIG_LOAD_INFO,
 		    spa->spa_load_info);
 	}