pinctrl: stm32: fix array read out of bound

pinctrl: stm32: fix array read out of bound

The existing code does not verify if the "tentative" index exceeds
the size of the array, causing out of bound read.
Issue identified with kasan.

Check the index before using it.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Fixes: 32c170ff15b0 ("pinctrl: stm32: set default gpio line names using pin names")
Link: https://lore.kernel.org/r/20231107110520.4449-1-antonio.borneo@foss.st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>