Crash when null texture parameter is present on a MIC

From UDN: [Link Removed]

Proposed fix:

int32 FHLSLMaterialTranslator::TextureParameter(FName ParameterName, UTexture* InDefaultValue, int32& TextureReferenceIndex, EMaterialSamplerType SamplerType, ESamplerSourceMode SamplerSource)
{
UTexture* DefaultValue = InDefaultValue;

// If we're compiling a function, give the function a chance to override the default parameter value
FMaterialParameterMetadata Meta;
//@CYA EDIT && nullptr != Meta.Value.Texture because we do not want to crash when calling DefaultValue->GetMaterialType(); below
if (GetParameterOverrideValueForCurrentFunction(EMaterialParameterType::Texture, ParameterName, Meta) && nullptr != Meta.Value.Texture)
//@CYA END

{ DefaultValue = Meta.Value.Texture; }