memtx: do not pass NULL to memcpy when creating gap item in MVCC

According to the C standard, passing `NULL` to `memcpy` is UB, even if
it copies nothing (number of bytes to copy is 0). The commit fixes such
situation in memtx MVCC.

Closes tarantool/security#129

NO_TEST=fix UB
NO_CHANGELOG=fix UB
NO_DOC=fix UB

(cherry picked from commit 24d38cef5adff900bea2484235762678ac1c5234)