MDEV-4529 Assertion `tmp->state == 4' fails on mix of INSTALL SONAME / UNINSTALL PLUGIN

fix incorrect assert

mysql-test/r/plugin.result

@@ -173,3 +173,22 @@ select 1;
 UNINSTALL PLUGIN example;
 UNINSTALL PLUGIN MyISAM;
 ERROR HY000: Built-in plugins cannot be deleted
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+plugin_name
+install soname 'ha_example';
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+plugin_name
+EXAMPLE
+UNUSABLE
+uninstall plugin example;
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+plugin_name
+UNUSABLE
+install soname 'ha_example';
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+plugin_name
+EXAMPLE
+UNUSABLE
+uninstall soname 'ha_example';
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+plugin_name

mysql-test/t/plugin.test

@@ -155,3 +155,17 @@ UNINSTALL PLUGIN example;
 --error ER_PLUGIN_DELETE_BUILTIN
 UNINSTALL PLUGIN MyISAM;
 
+#
+# MDEV-4529 Assertion `tmp->state == 4' fails on mix of INSTALL SONAME / UNINSTALL PLUGIN
+#
+
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+install soname 'ha_example';
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+uninstall plugin example;
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+install soname 'ha_example';
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+uninstall soname 'ha_example';
+select plugin_name from information_schema.plugins where plugin_library like 'ha_example%';
+

sql/sql_plugin.cc

@@ -2032,9 +2032,13 @@ static bool finalize_install(THD *thd, TABLE *table, const LEX_STRING *name)
                           ER_CANT_INITIALIZE_UDF, ER(ER_CANT_INITIALIZE_UDF),
                           name->str, "Plugin is disabled");
   }
+  else if (tmp->state != PLUGIN_IS_UNINITIALIZED)
+  {
+    /* already installed */
+    return 0;
+  }
   else
   {
-    DBUG_ASSERT(tmp->state == PLUGIN_IS_UNINITIALIZED);
     if (plugin_initialize(tmp))
     {
       report_error(REPORT_TO_USER, ER_CANT_INITIALIZE_UDF, name->str,