MAINT-6460 Crash calculating mesh complexity

indra/newview/llmeshrepository.cpp

@@ -4034,6 +4034,11 @@ void LLMeshRepository::uploadError(LLSD& args)
 //static
 F32 LLMeshRepository::getStreamingCost(LLSD& header, F32 radius, S32* bytes, S32* bytes_visible, S32 lod, F32 *unscaled_value)
 {
+	if (header.size() == 0)
+	{
+		return 0.f;
+	}
+
 	F32 max_distance = 512.f;
 
 	F32 dlowest = llmin(radius/0.03f, max_distance);