buffer: Don't cancel the backup in case the buffer is shared

Otherwise it will be removed async, which shouldn't happen in case there is
still one buffer open with the same modified file.

Author: JoeKar

internal/buffer/buffer.go

@@ -534,7 +534,9 @@ func (b *Buffer) Fini() {
 	if !b.Modified() {
 		b.Serialize()
 	}
-	b.CancelBackup()
+	if !b.Shared() {
+		b.CancelBackup()
+	}
 
 	if b.Type == BTStdout {
 		fmt.Fprint(util.Stdout, string(b.Bytes()))