backends/drm: check if m_commits is empty after waiting for commitPending

The main thread might've cleared the list, so it could crash

Fixes https://crash-reports.kde.org/share/issue/1e34fff2ef0d48f29fb64748caef884a/

src/backends/drm/drm_commit_thread.cpp

@@ -85,6 +85,9 @@ DrmCommitThread::DrmCommitThread(DrmGpu *gpu, const QString &name)
                     bool timeout = true;
                     while (std::chrono::steady_clock::now() < cursorTarget && timeout && m_commits.front()->isCursorOnly()) {
                         timeout = m_commitPending.wait_for(lock, 50us) == std::cv_status::timeout;
+                        if (m_commits.empty()) {
+                            break;
+                        }
                         optimizeCommits(cursorTarget);
                     }
                     if (!timeout) {
@@ -92,6 +95,9 @@ DrmCommitThread::DrmCommitThread(DrmGpu *gpu, const QString &name)
                         continue;
                     }
                 }
+                if (m_commits.empty()) {
+                    continue;
+                }
             }
             submit();
         }