lib: utils/irqchip: Add sanity checks in imsic_get_data() and imsic_get_target_file()

Add extra sanity checks to prevent the caller getting the invalid result from
imsic_get_data() or imsic_get_target_file() when imsic is not initialized
correctly.

Signed-off-by: Cyan Yang <cyan.yang@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>