boards native_posix: Detect attempt to configure not existing int

Similar to 66a4fe32. Prevent overrunning the irq vector table
in posix_irq_priority_set. This is not happening today in tree,
but coverity thinks it may. Checking for it to prevent it is not
a bad idea anyhow, so let's do it.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>