pinctrl: at91-pio4: check return value of devm_kasprintf()

In the Linux kernel, the following vulnerability has been resolved:

pinctrl: at91-pio4: check return value of devm_kasprintf()

devm_kasprintf() returns a pointer to dynamically allocated memory.

Pointer could be NULL in case allocation fails. Check pointer validity.

Identified with coccinelle (kmerr.cocci script).

Depends-on: 1c4e5c470a56 ("pinctrl: at91: use devm_kasprintf() to avoid potential leaks")

Depends-on: 5a8f9cf269e8 ("pinctrl: at91-pio4: use proper format specifier for unsigned int")