ftp: fix multi-thread copy

Before this change multi-thread copies using the FTP backend used to error with

    551 Error reading file

This was caused by a spurious error being reported which this code silences.

Fixes #7532
See #3942

fstest/test_all/config.yaml

@@ -298,10 +298,6 @@ backends:
    fastlist: false
  - backend:  "ftp"
    remote:   "TestFTPRclone:"
-   ignore:
-     - "TestMultithreadCopy/{size:131071_streams:2}"
-     - "TestMultithreadCopy/{size:131072_streams:2}"
-     - "TestMultithreadCopy/{size:131073_streams:2}"
    fastlist: false
  - backend:  "box"
    remote:   "TestBox:"

lib/readers/limited.go

@@ -1,6 +1,10 @@
 package readers
 
-import "io"
+import (
+	"io"
+
+	"github.com/rclone/rclone/fs"
+)
 
 // LimitedReadCloser adds io.Closer to io.LimitedReader.  Create one with NewLimitedReadCloser
 type LimitedReadCloser struct {
@@ -8,6 +12,16 @@ type LimitedReadCloser struct {
 	io.Closer
 }
 
+// Close closes the underlying io.Closer. The error, if any, will be ignored if data is read completely
+func (lrc *LimitedReadCloser) Close() error {
+	err := lrc.Closer.Close()
+	if err != nil && lrc.N == 0 {
+		fs.Debugf(nil, "ignoring close error because we already got all the data")
+		err = nil
+	}
+	return err
+}
+
 // NewLimitedReadCloser returns a LimitedReadCloser wrapping rc to
 // limit it to reading limit bytes. If limit < 0 then it does not
 // wrap rc, it just returns it.