Fix for BUG#12082 (assert failure when the query fails to get a lock for record in 'const' table):

 Set table->key_read back to 0 regardless of whether join_read_const() succeeded or not.

sql/sql_select.cc

@@ -9717,7 +9717,13 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
       table->file->extra(HA_EXTRA_KEYREAD);
       tab->index= tab->ref.key;
     }
-    if ((error=join_read_const(tab)))
+    error=join_read_const(tab);
+    if (table->key_read)
+    {
+      table->key_read=0;
+      table->file->extra(HA_EXTRA_NO_KEYREAD);
+    }
+    if (error)
     {
       tab->info="unique row not found";
       /* Mark for EXPLAIN that the row was not found */
@@ -9725,11 +9731,6 @@ join_read_const_table(JOIN_TAB *tab, POSITION *pos)
       if (!table->maybe_null || error > 0)
 	DBUG_RETURN(error);
     }
-    if (table->key_read)
-    {
-      table->key_read=0;
-      table->file->extra(HA_EXTRA_NO_KEYREAD);
-    }
   }
   if (*tab->on_expr_ref && !table->null_row)
   {