Check against integer overflow in int_array functions

Check against integer overflow in int_array functions

int_array_concat() and int_array_add_unique() could potentially end up
overflowing the int type variable used to calculate their length. While
this is mostly theoretical for platforms that use 32-bit int, there
might be cases where a 16-bit int overflow could be hit. This could
result in accessing memory outside buffer bounds and potentially a
double free when realloc() ends up freeing the buffer.

All current uses of int_array_add_unique() and most uses of
int_array_concat() are currently limited by the buffer limits for the
local configuration parameter or frame length and as such, cannot hit
this overflow cases. The only case where a long enough int_array could
be generated is the combination of scan_freq values for a scan. The
memory and CPU resource needs for generating an int_array with 2^31
entries would not be realistic to hit in practice, but a device using
LP32 data model with 16-bit int could hit this case.

It is better to have more robust checks even if this could not be
reached in practice, so handle cases where more than INT_MAX entries
would be added to an int_array as memory allocation failures instead of
allowing the overflow case to proceed.

Signed-off-by: Jouni Malinen <j@w1.fi>