MDEV-23092 SIGABRT when setting invalid wsrep_provider

Some invalid wsrep_provider paths may be interpreted as a valid
directory. For example '/invalid/libgalera_smm.so' with UTF character
set is interpreted as '/', which is a valid directory. A early check
that wsrep_provider should not be a directory fixes it.

mysql-test/suite/wsrep/r/MDEV-23092.result

+SET COLLATION_CONNECTION='utf16le_bin';
+SET GLOBAL wsrep_provider='/invalid/path/libgalera_smm.so';
+ERROR 42000: Variable 'wsrep_provider' can't be set to the value of '/'
+SET GLOBAL wsrep_cluster_address='OFF';
+SET GLOBAL wsrep_slave_threads=10;
+SELECT 1;
+1
+1
+SET GLOBAL wsrep_cluster_address='gcomm://';
+SET GLOBAL wsrep_slave_threads=DEFAULT;
+CALL mtr.add_suppression("wsrep_load()");
+CALL mtr.add_suppression("Failed to create a new provider");
+CALL mtr.add_suppression("Failed to load provider");

mysql-test/suite/wsrep/t/MDEV-23092.cnf

+!include ../my.cnf
+
+[mysqld.1]
+wsrep-on=OFF
+binlog-format=ROW
+wsrep-provider=none
+wsrep-cluster-address='gcomm://'
+innodb_autoinc_lock_mode=2

mysql-test/suite/wsrep/t/MDEV-23092.test

+#
+# MDEV-23092: SIGABRT in wsrep::server_state::provider when setting
+# invalid wsrep_provider (on optimized builds)
+#
+
+--source include/have_innodb.inc
+--source include/have_wsrep.inc
+--source include/have_binlog_format_row.inc
+
+SET COLLATION_CONNECTION='utf16le_bin';
+--error 1231
+SET GLOBAL wsrep_provider='/invalid/path/libgalera_smm.so';
+SET GLOBAL wsrep_cluster_address='OFF';
+SET GLOBAL wsrep_slave_threads=10;
+SELECT 1;
+
+SET GLOBAL wsrep_cluster_address='gcomm://';
+SET GLOBAL wsrep_slave_threads=DEFAULT;
+
+CALL mtr.add_suppression("wsrep_load()");
+CALL mtr.add_suppression("Failed to create a new provider");
+CALL mtr.add_suppression("Failed to load provider");

sql/wsrep_var.cc

@@ -320,6 +320,12 @@ static int wsrep_provider_verify (const char* provider_str)
   {
     return 1;
   }
+
+  if (MY_S_ISDIR(f_stat.st_mode))
+  {
+    return 1;
+  }
+
   return 0;
 }