ASoC: dapm: Fix NULL pointer dereference in snd_soc_dapm_free_kcontrol

w_text_param can be NULL and it is being dereferenced without checking.
Add the missing sanity check to prevent  NULL pointer dereference.

Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>