p2p/enode: add validation for binary.Varint/Uvarint in nodedb operations

[777genius]

Summary

This PR adds missing error checking for binary.Varint and binary.Uvarint calls in nodedb.go to ensure data integrity and prevent incorrect behavior when reading corrupted database entries.

Problem

Missing validation in two locations leads to inconsistency and potential issues:

1. Line 223 (fetchUint64): Ignores n return value - inconsistent with fetchInt64 which properly validates
2. Line 341 (expireNodes): Ignores n return value when reading pong timestamps

When binary.Varint/Uvarint returns n <= 0, it indicates:

• n == 0: Buffer too small (incomplete varint)
• n < 0: Value overflow (> 64 bits)

Without checks:

• fetchUint64() can return garbage values from corrupted data
• expireNodes() can use invalid timestamps leading to incorrect node expiration decisions
• Inconsistency between fetchInt64() (which validates) and fetchUint64() (which doesn't)

Solution

Added validation following the existing fetchInt64() pattern (lines 203-206):

val, n := binary.Uvarint(blob)
if n <= 0 {
    return 0
}
return val

For expireNodes(), added validation with silent skip:

time, n := binary.Varint(it.Value())
if n <= 0 {
    // Skip corrupted timestamp entry
    atEnd = !it.Next()
    continue
}

Testing

Added comprehensive tests:

• TestDBFetchUint64Corrupted: Tests truncated varint, overflow varint, empty data, and valid data cases
• TestDBExpireNodesCorrupted: Tests that expireNodes handles corrupted pong timestamps gracefully
• All existing tests pass

Context

This fixes an inconsistency where fetchInt64() at lines 203-206 properly validates the return value but fetchUint64() does not.