The special case with NULL as a regular expression
was handled at prepare time. But in this special case
the item was not marked as fixed. This caused an assertion
at execution time.
Fixed my marking the item as fixed even when known to 
return NULL at prepare time.


mysql-test/r/func_regexp.result:
  Bug #31440: test case
mysql-test/t/func_regexp.test:
  Bug #31440: test case
sql/item_cmpfunc.cc:
  Bug #31440: mark the item as fixed even when 
  known to return NULL.