devm_kasprintf() can return a NULL pointer on failure but this returned
value is not checked. Fix this lack and check the returned value.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: 32c170ff ("pinctrl: stm32: set default gpio line names using pin names")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Link: https://lore.kernel.org/20240906100326.624445-1-make24@iscas.ac.cnSigned-off-by: Linus Walleij <linus.walleij@linaro.org>