memtx: fix size calculation in functional index chunks

When function of a functional index is called, the result is
stored in memtx allocator chunks among data tuples. By design,
memtx allocator requires size of allocation for deallocation,
so the size has to be stored along with the data (actually right
before it). By a mistake, when being deleted, the size of data
was retrieved slightly wrong, giving the value of 4 bytes less.
Due to the allocator specific design the size error leads to a
rare crashes when the size of functional index function result
was about 160 bytes (157..160 bytes with default config). It
seems that sizes about 320 etc are affected to.

Fix it by correct size evaluation of functional index chunks.

Hotfix of #6786

NO_DOC=bug fix
NO_TEST=see later commits