Invalid addrs crushes sc-iterator methods usages

[NikitaZotov]

Describe the bug
If sc-iterator was created with invalid not-exist addrs, then its methods is dangerous for usage, because they call errors.

To Reproduce
Steps to reproduce the behavior:

1. Create non-valid sc-address, for example ScAddr const & addr = ScAddr(125);;
2. Create 3-constr iterator `ScIterator3Ptr const iter3 = m_ctx.Iterator3(addr, ..., ...);
3. Call Next and Get methods from it.

Expected behavior
Iterators must check themself. It can be solved by IsValid() calls at the begin of iterators methods.

Desktop (please complete the following information):

• OS: [e.g. Ubuntu 20.04]
• Version [e.g. 0.6.1]