fix: Add null check (#7305)

Signed-off-by: Pratik Mankawde <3397372+pratikmankawde@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/xrpld/rpc/handlers/orderbook/GetAggregatePrice.cpp

@@ -51,7 +51,6 @@ iteratePriceData(
     std::shared_ptr<SLE const> const& sle,
     std::function<bool(STObject const&)> const& f)
 {
-    using Meta = std::shared_ptr<STObject const>;
     static constexpr std::uint8_t kMaxHistory = 3;
     bool isNew = false;
     std::uint8_t history = 0;
@@ -73,7 +72,7 @@ iteratePriceData(
     // for the Oracle is not found in the inner loop
     STObject const* prevChain = nullptr;
 
-    Meta meta = nullptr;
+    std::shared_ptr<STObject const> meta = nullptr;
     while (true)
     {
         if (prevChain == chain)
@@ -93,6 +92,8 @@ iteratePriceData(
             return;  // LCOV_EXCL_LINE
 
         meta = ledger->txRead(prevTx).second;
+        if (!meta)
+            return;
 
         prevChain = chain;
         for (STObject const& node : meta->getFieldArray(sfAffectedNodes))