Fix binding of reference to null pointer in RefFullyConnectedWorkload

As the bias decoder is optional, pass it as a pointer to the FullyConnected
function instead of by reference.

Signed-off-by: Matthew Bentham <matthew.bentham@arm.com>
Change-Id: I0bd2e601015e7fa1ae21b50f6c3dd7eac4176554
diff --git a/src/backends/reference/workloads/RefFullyConnectedWorkload.cpp b/src/backends/reference/workloads/RefFullyConnectedWorkload.cpp
index 3e63afa..99e3eab 100644
--- a/src/backends/reference/workloads/RefFullyConnectedWorkload.cpp
+++ b/src/backends/reference/workloads/RefFullyConnectedWorkload.cpp
@@ -102,7 +102,7 @@
                    *OutputEncoder,
                    m_WeightShape,
                    *m_WeightDecoder,
-                   *m_BiasDecoder,
+                   m_BiasDecoder.get(),
                    m_Data.m_Parameters.m_BiasEnabled,
                    m_NumActivations,
                    m_Data.m_Parameters.m_TransposeWeightMatrix);