ext/ftp: fix off-by-one terminator write in ftp_readline()#22377
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The bug80901 fix (09696ee) terminates an over-long response with *data = 0, but when the line fills the whole FTP_BUFSIZE inbuf without a CR/LF, data points at inbuf[FTP_BUFSIZE] and the terminator is written one byte past the buffer, into the adjacent ftpbuf_t::extra field. Reserve the final byte for the terminator so it always lands inside inbuf. A buffer-filling response loses its last character (bug80901's SYST reply is now 4095 visible chars, with the terminator taking the 4096th slot).
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A response that fills FTP_BUFSIZE with no CR/LF makes ftp_readline() write its NUL terminator one byte past inbuf, into the adjacent ftpbuf_t::extra field. Reserve the final byte for the terminator so it always lands inside inbuf. Regression from the bug80901 fix (09696ee); the SYST reply in that test is now 4095 visible characters.