Actually it is not a bug but right behavior observed as pefomance 
degradation after we have forced Item_field::fix_fields() to 
re-execute each time when we are executing prep stmt.

This patch implements small optimization which heals this bad 
behavior. We are caching field position in TABLE::field array in
Item's member and are using this position for speeding up field
lookups in fix_fields() in case of its re-execution.