Fix GH-10562: Memory leak and invalid state with consecutive ftp_nb_fget

When the user does not fully consume the data stream, but instead opens
a new one, a memory leak occurs. Moreover

Fix GH-10562: Memory leak and invalid state with consecutive ftp_nb_fget

When the user does not fully consume the data stream, but instead opens
a new one, a memory leak occurs. Moreover, the state is invalid: when
more commands arrive they'll be handled out-of-sync because the state of
the client does not match what the server is doing.
This leads to all sorts of weirdness, for example:
Warning: ftp_nb_fget(): OK.

Fix it by gracefully closing the old data stream when a new data stream
is started.

Closes GH-11606.

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