misc: avoid allocations of size 0 for region

Regular region implementation supports allocations of size 0 with no
extra efforts. It returns a non-NULL pointer in this case. However in
case of ASAN friendly implementation it will require a special care for
this case. Instead let's avaid allocations if size 0 for region.

Also use xregion_ macros for allocations. Our current policy is to panic
on OOM on runtime allocations.

Part of tarantool/tarantool#7327

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