LOCAL TABLE WHEN ONLY 1 LOCAL ROW

Description: When updating a federated table with UPDATE...
JOIN, the server consistently crashes with Signal 11 when
only 1 row exists in the local table involved in the join 
and that 1 row can be joined with a row in the federated 
table.

Analysis: Interaction between the federated engine and the
optimizer results in the crash. In our scenario, ie, local
table having only one row, the program is following a 
different path because the table is treated as a constant
table by the join optimizer. So in this scenario 
"index_read()" is happening in the prepare phase,
since optimizer plan is different for constant table joins.
In this case, "index_read_idx_map()" (inside handler.cc) is
calling "index_read()" and inside "index_read()", matching 
rows are fetched and "stored_result" gets populated by 
calling "store_result()". And just after "index_read()", 
"index_end()" function is called. And in the "index_end()",
its freeing the "stored_result" by calling "free_result()".
So when it reaches the execution phase, in "position()" 
function, we are getting assertion at 
"DBUG_ASSERT(stored_result);". In all other scenarios (ie, 
table with more than 1 row), optimizer plan is different 
and "index_read()" is happening in the execution phase.

Fix: So my fix is to have a separate ha_federated member
function for "index_read_idx_map()" which will handle 
federated engine separately. So that position() will be 
called before index_end() call in constant table scenario.