Preserve enforced masternode sync failures

[NateIsern]

Motivation

• The unconditional stall timeout in CMasternodeSync::Process() could advance sync stages (LIST->MNW->BUDGET->FINISHED) even when no masternode list or winner data was received, allowing a node to mark itself synced with empty/incomplete masternode/payment state.
• A falsely finished masternode sync can unblock PoS staking (masternodeSync.IsSynced()) and cause payment validation to "fail open", creating the risk of producing or accepting blocks inconsistent with the network when SPORK_8_MASTERNODE_PAYMENT_ENFORCEMENT is active.

Description

• Change src/masternode-sync.cpp so the global stall timeout preserves the SPORK_8_MASTERNODE_PAYMENT_ENFORCEMENT failure semantics: when the timeout fires and the current asset is MASTERNODE_SYNC_LIST or MASTERNODE_SYNC_MNW with no data (lastMasternodeList == 0 or lastMasternodeWinner == 0) and the spork is active, the code now sets RequestedMasternodeAssets = MASTERNODE_SYNC_FAILED and records the failure instead of blindly calling GetNextAsset().
• Keep the original recovery behavior for empty/small networks by still advancing assets via GetNextAsset() when payment enforcement is inactive or when relevant data has been received.
• The change is local to CMasternodeSync::Process() and does not modify consensus or block-validation rules.

Testing

• Ran git diff --check on the modified tree and there were no whitespace or diff-check issues (passed).
• Attempted a full build with ./autogen.sh && ./configure --without-gui --disable-wallet --disable-tests && make -j2 but the build could not be executed in this environment because aclocal (autotools) is missing, so a full compile/test was not completed.

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Codex Task