sql: fix assert when compare a tuple with a key

The function of the question (`tarantoolsqlIdxKeyCompare()`) compares a
tuple with a key. It extracts necessary fields from the tuple and runs
the compare function, which is basically `memcmp()` of two buffers (for
`varbinary` values -- numeric comparisons are different).

In the Debug build the function also re-verifies the result using a
separate comparison code. And here we compare two `memcmp()` return
values.

We shouldn't do it directly, because `memcmp()` only guarantees sign of
the result. Please, consider the linked issue for details.

NO_DOC=only debug build is affected
NO_CHANGELOG=only debug build is affected
NO_TEST=hard to create a stable reproducer

Fixes #6849