In 37553 we declared longlong results for
class Item_str_timefunc as per comments/docs,
but didn't add a method for that. And the
default just wasn't good enough for some
cases.

Changeset adds dedicated val_int() to class.

mysql-test/r/func_sapdb.result:
  More tests for casts of TIME() / TIMEDIFF() with negative results.
mysql-test/t/func_sapdb.test:
  More tests for casts of TIME() / TIMEDIFF() with negative results.
sql/item_timefunc.h:
  Since we claim to provide longlong results, we should have
  a suitable function to provide them (the default won't do).
  This one matches the val_real() variant.