ASoC: simple-card-utils: Fix pointer check in graph_util_parse_link_direction

ASoC: simple-card-utils: Fix pointer check in graph_util_parse_link_direction

Actually check if the passed pointers are valid, before writing to them.
This also fixes a USBAN warning:
UBSAN: invalid-load in ../sound/soc/fsl/imx-card.c:687:25
load of value 255 is not a valid value for type '_Bool'

This is because playback_only is uninitialized and is not written to, as
the playback-only property is absent.

Fixes: 844de7eebe97 ("ASoC: audio-graph-card2: expand dai_link property part")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://patch.msgid.link/20250429094910.1150970-1-alexander.stein@ew.tq-group.com
Signed-off-by: Mark Brown <broonie@kernel.org>