sstables: mc: index_reader: Handle CK_SIZE split across buffers properly
sstables: mc: index_reader: Handle CK_SIZE split across buffers properly

we incorrectly falled-through to the next state instead of returning
to read more data.

This can manifest in a number of ways, an abort, or incorrect read.

Introduced in 917528c

Fixes #4011.

Message-Id: <ac915394b7142318fd8b627e56efcee0e65163b4@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla