Variant#3: moved the logic out of create_key_parts_for_pseudo_indexes

Range Analyzer (get_mm_tree functions) can only process up to MAX_KEY=64
indexes. The problem was that calculate_cond_selectivity_for_table used
it to estimate selectivities for columns, and since a table can
have > MAX_KEY columns, would invoke Range Analyzer with more than MAX_KEY
"pseudo-indexes".

Fixed by making calculate_cond_selectivity_for_table() to run Range
Analyzer with at most MAX_KEY pseudo-indexes. If there are more
columns to process, Range Analyzer will be invoked multiple times.

Also made this change:
-    param.real_keynr[0]= 0;
+    MEM_UNDEFINED(&param.real_keynr, sizeof(param.real_keynr));

Range Analyzer should have no use on real_keynr when it is run with
pseudo-indexes.