Fix a syncFile panic when LSS not opened in synchronous mode (config.SyncInterval set)

--- FAIL: TestLSSFlushBufferSizePersist128K (0.11s)

panic: runtime error: invalid memory address or nil pointer dereference [recovered]

        panic: runtime error: invalid memory address or nil pointer dereference

[signal SIGSEGV: segmentation violation code=0x1 addr=0x78 pc=0x100850162]        /Users/saptarshi.sen/.cbdepscache/exploded/x86_64/go-1.20.4/go/src/runtime/panic.go:884 +0x213

github.com/couchbase/plasma.syncFile({0x0, 0x0})

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/util.go:666 +0x22

github.com/couchbase/plasma.(*multiFilelog).growLog(0xc000478000)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/log.go:406 +0x132

github.com/couchbase/plasma.(*multiFilelog).Append(0xc000478000, {0xc000220000?, 0x1000, 0xc0004669e8?})

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/log.go:448 +0x130

github.com/couchbase/plasma.(*lsStore).flush(0xc0004726c0, 0xc000528080)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/lss.go:290 +0x132

github.com/couchbase/plasma.(*flushBuffer).Done(0xc0004726c0?)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/lss.go:1257 +0x84

github.com/couchbase/plasma.(*lsStore).Sync(0xc0004726c0, 0x1)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/lss.go:659 +0x77

github.com/couchbase/plasma.(*LSSCtx).LSSSync(...)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/lssctx.go:457

github.com/couchbase/plasma.(*Plasma).Sync(0xc0003c4000, 0xe0?)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/plasma.go:1990 +0x35

github.com/couchbase/plasma.(*Plasma).updateMaxSn(0xc0003c4000, 0x1, 0x20?)

        /Users/saptarshi.sen/ws/couchbase/elixir/goproj/src/github.com/couchbase/plasma/mvcc.go:1040 +0x218

github.com/couchbase/plasma.(*Plasma).doInit(0xc0003c4000)