Check the return value of query.exec in dbutil.cpp

Fixes coverity 703634
(cherry picked from commit c45db22)

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>

Fixes #11567

mythtv/libs/libmythbase/dbutil.cpp

@@ -863,7 +863,10 @@ bool DBUtil::TryLockSchema(MSqlQuery &query, uint timeout_secs)
 void DBUtil::UnlockSchema(MSqlQuery &query)
 {
     query.prepare("SELECT RELEASE_LOCK('schemaLock')");
-    query.exec();
+    if (!query.exec())
+    {
+        MythDB::DBError("DBUtil UnlockSchema", query);
+    }
 }
 
 /** \fn CheckTimeZoneSupport(void)