Bug #30648: Partition handler may not initialize variable used w/ autoincrement

A local variable may be used uninitialized in 
ha_partition::get_auto_increment().  Initialize it properly.

sql/ha_partition.cc

@@ -5445,6 +5445,7 @@ void ha_partition::get_auto_increment(ulonglong offset, ulonglong increment,
 
   for (pos=m_file, end= m_file+ m_tot_parts; pos != end ; pos++)
   {
+    first_value_part= *first_value;
     (*pos)->get_auto_increment(offset, increment, nb_desired_values,
                                &first_value_part, &nb_reserved_values_part);
     if (first_value_part == ~(ulonglong)(0)) // error in one partition