buffer: Don't Serialize in case the buffer is shared

Otherwise we do unnecessarily serialize per shared buffer as well as storing
multiple cursor states, while only the last one will be restored anyway.

Author: JoeKar

internal/buffer/buffer.go

@@ -526,10 +526,10 @@ func (b *Buffer) Close() {
 // Fini should be called when a buffer is closed and performs
 // some cleanup
 func (b *Buffer) Fini() {
-	if !b.Modified() {
-		b.Serialize()
-	}
 	if !b.Shared() {
+		if !b.Modified() {
+			b.Serialize()
+		}
 		b.CancelBackup()
 	}