The RAND(N) function where the N is a field of "constant" table
(table of single row) failed with a SIGFPE.

Evaluation of RAND(N) rely on constant status of its argument.
Current server "seeded" random value for each constant argument
only once, in the Item_func_rand::fix_fields method.
Then the server skipped a call to seed_random() in the
Item_func_rand::val_real method for such constant arguments.

However, non-constant state of an argument may be changed
after the call to fix_fields, if an argument is a field of
"constant" table. Thus, pre-initialization of random value
in the fix_fields method is too early.


Initialization of random value by seed_random() has been
removed from Item_func_rand::fix_fields method.
The Item_func_rand::val_real method has been modified to
call seed_random() on the first evaluation of this method
if an argument is a function.


mysql-test/r/func_math.result:
  Added test case for bug #44768.
mysql-test/t/func_math.test:
  Added test case for bug #44768.
sql/item_func.cc:
  Bug #44768: SIGFPE crash when selecting rand from a view containing null
  
  1. Initialization of random value by seed_random() has been
     removed from Item_func_rand::fix_fields method.
  2. The Item_func_rand::val_real method has been modified to
     call seed_random() on the first evaluation of this method
     if an argument is a function.
sql/item_func.h:
  Bug #44768: SIGFPE crash when selecting rand from a view containing null
  
  1. The Item_func_rand::first_eval has been added to trace
     the first evaluation of the val_real method.
  2. The Item_func_rand::cleanup method has been added to
     cleanup the first_eval flag.