gen_isr_tables: Added check of the IRQ num before accessing the vt

At its current state, the script tries to access the vector table
list without checking first that the index is valid. This can
cause the script to crash without a descriptive message.
The index can be invalid if an IRQ number that is larger than
the maximum number allowed by the SOC is used.
This PR adds a check of that index, that exits with an error
message if the index is invalid.

Fixes #29809

Signed-off-by: Yonatan Schachter <yonatan.schachter@gmail.com>