Change QCborArray to pad with invalid on inserting past end

Likewise have mutating operator[] insert an invalid entry at its
target index, if beyond the end of the array. This makes it possible
to fill an array from high index to low, for example.

Change-Id: If71699c20e2623142214ce2c11c4d6e4a120c989
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>