altoslib: Fix data fetching during flashing of cc1111-based devices

altoslib: Fix data fetching during flashing of cc1111-based devices

We want to get enough information about the target device to verify
that the new firmware matches, so fetch 512 bytes instead of just
barely enough to cover the romconfig data.

Also catch out-of-bounds accesses and handle them, in case even this
large array isn't enough.

Signed-off-by: Keith Packard <keithp@keithp.com>