fix: avoid nil-pointer panic when store.New returns an error

[piyush295]

Fixes #288.

Current behavior

Registry.Get calls store.New(...) but does not check its result before use:

session, err = store.New(s.request, name)
session.name = name   // nil-pointer dereference if store.New returned (nil, err)

A Store whose New returns (nil, err) — which the interface permits — causes a nil-pointer panic at session.name = name (and again at session.store = store), instead of the error being returned to the caller.

Fix

Guard against a nil session: if store.New returns nil, return the error immediately rather than dereferencing it.

session, err = store.New(s.request, name)
if session == nil {
    return nil, err
}
session.name = name
...

Testing

• Added TestRegistryGet_StoreNewError: a store whose New returns (nil, err) now yields a nil session + error from Get instead of panicking.
• go test . — full suite passes; gofmt clean; go build . clean.