A helper method encasuplating the asserts related to index into the
container and length, modelled after the QVLA::verify().
`pos <= size` is OK because if pos == size, the e.g. sliced()'ed
container is just going to be empty.
Normalize how verify() is used, the first arg is an index and the second
a length.
This method is constexpr even in QString/QByteArray merely for
consistency with similar methods in other string classes (this
necessitates using `d.size` in verify() in QString/QBA because size()
isn't constexpr).
Change-Id: I90e3c56d76c802259297a06d11d46ee342a1daf2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
2fd9735e7a