vinyl: update mem ptr in vy_build_insert_tuple() after yield

vy_build_insert_tuple() processes insertion into secondary indexes being
created. It contains yield points during which in-memory level of LSM
tree may change (for example rotate owing to triggered dump). So after
yield point it is required to fetch from LSM struct pointer to mem again
to operate on valid metadata. This patch updates pointer to mem after
mentioned yield point.

Closes #4810