ftp: don't let first incomplete segment be over maximum length
The first non-terminated ftp command segment is buffered with no size limit. This results in a subsequent segment causing an integer to enter a negative state which is then asserted on with a
DEBUG_VALIDATE_BUG_ON. Make sure the first segment is subject to limits of subsequent segments.
Then result in non-debug-validate-bug-on builds is that the first logged segment can be up to 65k, but their should be no security related issue here as the right thing is already done for negative values.