libs: context: use attribs index instead pointers

libs: context: use attribs index instead pointers

Coverity scan bug:

Out-of-bounds write. This could cause an immediate crash or incorrect
computations.

Coverity basically found that it is possible to assign more than 4
attribs in the array.

In my opinion this was produced because code pattern used pointer
arithmetic, which is not readable nor maintainable.

This patch refactors config_create() to use an array index rather than
pointer arithmetic. Also a run-time check for index size was added.