str_to_date function should only try to generate a warning for
invalid input strings, not when input value is NULL. In latter
case, val_str() of input argument will return a nil pointer.
Trying to generate a warning using this pointer lead to a
segmentation fault. Solution: Only generate warning when pointer
to input string is non-nil.


mysql-test/r/func_time.result:
  Added test case for Bug#57512
mysql-test/t/func_time.test:
  Added test case for Bug#57512
sql/item_timefunc.cc:
  Skip generating warning when pointer to input string is nil
  since this implies that input argument was NULL.