ftp: support growing files with CURLOPT_IGNORE_CONTENT_LENGTH

ftp: support growing files with CURLOPT_IGNORE_CONTENT_LENGTH

When using the option CURLOPT_IGNORE_CONTENT_LENGTH (set.ignorecl in
code) to support growing files in FTP, the code should ignore the
initial size it gets from the server as this will not be the final size
of the file. This is done in ftp_state_quote() to prevent a size request
being issued in the initial sequence. However, in a later call to
ftp_state_get_resp() the code attempts to get the size of the content
again if it doesn't already have it, by parsing the response from the
RETR request. This fix prevents this parsing of the response to get the
size when the set.ignorecl option is set. This should maintain the size
value as -1, unknown, in this situation.

Closes #9772