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.

Tracker issue: https://tracker.ceph.com/issues/61503

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>

src/librbd/io/SimpleSchedulerObjectDispatch.cc

@@ -105,7 +105,8 @@ bool SimpleSchedulerObjectDispatch<I>::ObjectRequests::try_delay_request(
 
     // try to merge back to an existing request
     iter = m_delayed_requests.lower_bound(object_off);
-    if (iter == m_delayed_requests.end() || iter->first > object_off) {
+    if (iter != m_delayed_requests.begin() &&
+        (iter == m_delayed_requests.end() || iter->first > object_off)) {
       iter--;
     }
     if (iter != m_delayed_requests.end() &&