box: fix use-after-free in space_truncate

space_truncate allocates a statement on the stack which is grossly
incorrect as the stack may be purged once the function returns while
box_process_rw expects the statement to be valid until the end of
the transaction. By happy accident, it worked fine until commit
1f7b0d65 ("Require for single statement not autocommit in case of
ddl"), which made it possible to run this function from a transaction
and hence increased the probability of hitting the use-after-free bug.
The fix is trivial: allocate a truncation statement on the region.

Fixes commit 353bcdc5 ("Rework space truncation").

Closes #4093