Fix memory leak in nanoftp

Found by OSS-Fuzz (testcase 5487741645094912). Free the user and
password strings in xmlNanoFTPFreeCtxt.

Also see https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=2727
diff --git a/nanoftp.c b/nanoftp.c
index 2135ab9..5bc6555 100644
--- a/nanoftp.c
+++ b/nanoftp.c
@@ -511,6 +511,8 @@
     if (ctxt->hostname != NULL) xmlFree(ctxt->hostname);
     if (ctxt->protocol != NULL) xmlFree(ctxt->protocol);
     if (ctxt->path != NULL) xmlFree(ctxt->path);
+    if (ctxt->user != NULL) xmlFree(ctxt->user);
+    if (ctxt->passwd != NULL) xmlFree(ctxt->passwd);
     ctxt->passive = 1;
     if (ctxt->controlFd != INVALID_SOCKET) closesocket(ctxt->controlFd);
     ctxt->controlFd = INVALID_SOCKET;