mutation_reader: do not ask for mutation before current is consumed
mutation_reader: do not ask for mutation before current is consumed

mutation_reader and streamed_mutation may use the same stream as a source
mutation_fragments and mutations themselves (this happens in sstable reader).
In such case asking for next streamed_mutation from mutation_reader would
invalidate all other streamed_mutations.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,raphaelsc/scylla