libcurl's TFTP state machine fails to validate state transitions against the
requested transfer direction, accepting opening server responses for both
reads and writes interchangeably. A malicious TFTP server can respond to a
download request in a way that forces the client into transmit mode, prompting
libcurl to consume and transmit data from the handle's persistent
CURLOPT_READDATA pointer - including data remaining from unrelated prior
operations, such as an HTTP POST request executed on a reused handle.