Unmapping an unmapped buffer should succeed

Hardware tests show that trying to unmap an unmapped buffer already should always succeed. Hardware test was tested up to 32 iterations of attempting to unmap

src/core/hle/service/nvdrv/devices/nvhost_as_gpu.cpp

@@ -157,7 +157,12 @@ u32 nvhost_as_gpu::UnmapBuffer(const std::vector<u8>& input, std::vector<u8>& ou
     LOG_DEBUG(Service_NVDRV, "called, offset=0x{:X}", params.offset);
 
     const auto itr = buffer_mappings.find(params.offset);
-    ASSERT_MSG(itr != buffer_mappings.end(), "Tried to unmap invalid mapping");
+    if (itr == buffer_mappings.end()) {
+        LOG_WARNING(Service_NVDRV, "Tried to unmap an invalid offset 0x{:X}", params.offset);
+        // Hardware tests shows that unmapping an already unmapped buffer always returns successful
+        // and doesn't fail.
+        return 0;
+    }
 
     auto& system_instance = Core::System::GetInstance();