memtx: fix TREE index `get` check for part count

If TREE index `get` result is empty, the key part count is incorrectly
compared to the tree's `cmp_def->part_count`, though it should be compared
with `cmp_def->unique_part_count`. But we can actually assume that by the
time we get to the index's `get` method the part count is equal to the
unique part count (partial keys are rejected and `get` is not
supported for non-unique indexes): change check to correct assertion.

Closes #7685

NO_DOC=<bugfix>

(cherry picked from commit bfcd8ca7)