fix: Guard against null SLE in NoRippleCheck and GatewayBalances callbacks

[SAY-5]

High Level Overview of Change

The forEachItemAfter callback in doNoRippleCheck and the forEachItem callback in doGatewayBalances dereference the SLE argument (ownedItem/sle) without checking for null first. The sibling account handlers (doAccountLines, doAccountChannels, doAccountOffers) all guard their directory callbacks with a if (!sle) { UNREACHABLE(...); return; } block. This adds the same guard to the two handlers that were missing it, so all directory-iteration callbacks are consistent. Fixes #7510.

Context of Change

Both callbacks receive the SLE via view.read() inside the directory helpers. In normal operation read() returns a valid entry, but the other account handlers already treat a null result as an unreachable-but-guarded case rather than dereferencing it. This change closes that inconsistency; the guard uses the existing UNREACHABLE macro with LCOV_EXCL_START/LCOV_EXCL_STOP markers, matching AccountLines.cpp.

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

Test Plan

The null-SLE path is defensive and not reachable through the existing RPC unit tests without a corrupted ledger, so it is marked with LCOV_EXCL like the equivalent guard in AccountLines.cpp. The existing NoRippleCheck and GatewayBalances test suites exercise the normal (non-null) path and continue to pass. The change is formatted with the repository .clang-format.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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