Fixes asyncftpclient multiline reading, fixes #4684 (#13242)

Previously, the 4th character of `result` was checked for `'-'` every time, instead of each new line.

Also made it work for taint mode.

lib/pure/asyncftpclient.nim

@@ -131,12 +131,13 @@ type
 const multiLineLimit = 10000
 
 proc expectReply(ftp: AsyncFtpClient): Future[TaintedString] {.async.} =
-  result = await ftp.csock.recvLine()
+  var line = await ftp.csock.recvLine()
+  result = TaintedString(line)
   var count = 0
-  while result[3] == '-':
+  while line[3] == '-':
     ## Multi-line reply.
-    let line = await ftp.csock.recvLine()
-    result.add("\n" & line)
+    line = await ftp.csock.recvLine()
+    string(result).add("\n" & line)
     count.inc()
     if count >= multiLineLimit:
       raise newException(ReplyError, "Reached maximum multi-line reply count.")
@@ -178,7 +179,7 @@ proc connect*(ftp: AsyncFtpClient) {.async.} =
   await ftp.csock.connect(ftp.address, ftp.port)
 
   var reply = await ftp.expectReply()
-  if reply.startsWith("120"):
+  if string(reply).startsWith("120"):
     # 120 Service ready in nnn minutes.
     # We wait until we receive 220.
     reply = await ftp.expectReply()