Use llrint() rather than (longlong)rint() in Item_func_numhybrid::val_int()
Check for out-of-bounds values before converting double to longlong.

Ensure that instances of Query_result_union_direct have a
SELECT_LEX_UNIT to a avoid taking the address of a nullptr.

Be more careful when converting float/double to (unsigned) long long,
to avoid possible under/overflow.

Fix function argument to tree_walk, don't do C-style casts to force an
interface.

Change-Id: Ib9a8245291cc1e4ac1b3297d362ca70e9d4c55c5