devel/indi: unbreak the port's build on 32-bit architectures

Use of overloaded operator [] is ambiguous (with operand types INDI::
PropertyNumber and int) because candidate function ``WidgetView<T>
&operator[](size_t index) const'' conflicts with built-in candidate
operator. Use signed type with an assertion to avoid the clash.

On 64-bit machines size_t' is 64-bit but int' is still 32-bit so
there's no clash.

Obtained from: upstream (partially)