librbd: avoid decrementing iterator before first element

librbd: avoid decrementing iterator before first element

While trying to merge delayed requests, SimpleSchedulerObjectDispatch
can end up iterating before the first element.

With llvm, this leads to a crash:
https://paste.opendev.org/raw/bxnQqqDtIrkOfVvRfkZ6/

This change adds a check, ensuring that we won't decrement the iterator
before the first map element.

Fixes: https://tracker.ceph.com/issues/61503
Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>