Fix null pointer dereferences in PAM

Coverity caught these.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#13889

contrib/pam_zfs_key/pam_zfs_key.c

@@ -531,7 +531,6 @@ zfs_key_config_get_dataset(zfs_key_config_t *config)
 		if (zhp == NULL) {
 			pam_syslog(NULL, LOG_ERR, "dataset %s not found",
 			    config->homes_prefix);
-			zfs_close(zhp);
 			return (NULL);
 		}
 
@@ -543,6 +542,10 @@ zfs_key_config_get_dataset(zfs_key_config_t *config)
 		return (dsname);
 	}
 
+	if (config->homes_prefix == NULL) {
+		return (NULL);
+	}
+
 	size_t len = ZFS_MAX_DATASET_NAME_LEN;
 	size_t total_len = strlen(config->homes_prefix) + 1
 	    + strlen(config->username);