[lldb/Reproducers] Assert when trying to get object for invalid index.

Assert when trying to get an object for an index we haven't seen before.
This will crash anyway, the assertion is just a bit more informative.

lldb/source/Utility/ReproducerInstrumentation.cpp

@@ -15,6 +15,7 @@ using namespace lldb_private;
 using namespace lldb_private::repro;
 
 void *IndexToObject::GetObjectForIndexImpl(unsigned idx) {
+  assert(m_mapping.count(idx) && "No object for index");
   return m_mapping.lookup(idx);
 }