Problem: REGEXP in functions/PSs may return wrong results
due to improper initialization.

Fix: initialize required REGEXP params.


sql/item_cmpfunc.cc:
  Fix for bug#37337: Function returns different results
  
  prev_regexp is used in the Item_func_regex::regcomp()
  to store previous regex and to avoid re-initialization 
  if given the same pattern.
  Shoud be deleted in the Item_func_regex::cleanup() where we
  clean up the regexp structure.