Debugger: Remove a wrong soft assert in breakpoint handler

The code path can be triggered legitimately by removing breakpoints
without direct file association like "Break on throw" in the
breakpoint view.

Change-Id: If32a990f76d86386e40123c8f7a90613e15ca670
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/debugger/breakhandler.cpp

@@ -1329,11 +1329,12 @@ BreakpointItem::~BreakpointItem()
 
 void BreakpointItem::destroyMarker()
 {
-    BreakpointMarker *m = m_marker;
-    QTC_ASSERT(m, return);
-    m->m_bp = 0;
-    m_marker = 0;
-    delete m;
+    if (m_marker) {
+        BreakpointMarker *m = m_marker;
+        m->m_bp = 0;
+        m_marker = 0;
+        delete m;
+    }
 }
 
 QString BreakpointItem::markerFileName() const