handling.

The problem was that parsing of nested regular expression involved
recursive calls. Such recursion didn't take into account the amount of
available stack space, which ended up leading to stack overflow crashes.

mysql-test/t/not_embedded_server.test:
  Added test for bug#58026.
regex/my_regex.h:
  added pointer to function as last argument of my_regex_init() for check
  enough memory in stack.
regex/regcomp.c:
  p_ere() was modified: added call to function for check enough memory
  in stack. Function for check available stack space specified by
  global variable my_regex_enough_mem_in_stack. This variable set to
  NULL for embedded mysqld and to a pointer to function
  check_enough_stack_size otherwise.
regex/reginit.c:
  my_regex_init was modified: pass a pointer to a function for check
  enough memory in stack space. Reset this pointer to NULL in my_regex_end.
sql/mysqld.cc:
  Added function check_enough_stack_size() for check enough memory in stack.
  Passed this function as second argument to my_regex_init. For embedded 
  mysqld passed NULL as second argument.