pipeline: fix packet mirroring configuration

pipeline: fix packet mirroring configuration

Fix segmentation fault due to null pointer dereferencing inside the
"mirror" instruction when number of mirroring slots is set to 0. This
was taking place when the "mirror" instruction was used without the
mirror feature being properly configured, i.e. the API function
rte_swx_pipeline_mirroring_config was not called at initialization.

Fixes: dac0ecd9098 ("pipeline: support packet mirroring")
Signed-off-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>