Alexey Simak found out that when doing FTP with the multi interface and

something went wrong like it got a bad response code back from the server,
libcurl would leak memory. Added test case 538 to verify the fix.

I also noted that the connection would get cached in that case, which
doesn't make sense since it cannot be re-use when the authentication has
failed. I fixed that issue too at the same time, and also that the path
would be "remembered" in vain for cases where the connection was about to
get closed.