FtpServer.cc:1024: "reply != NULL" assertion

FtpServer.cc:1024: "reply != NULL" assertion

Handle nil HttpReply pointer inside various handlers called from
Ftp::Server::handleReply(). For example, when the related StoreEntry
object is aborted, the client_side_reply.cc code may call the
Ftp::Server::handleReply() method with a nil reply pointer.

The Ftp::Server::handleReply() methods itself cannot handle nil replies
because they are valid in many states. Only state-specific handlers know
whether they need the reply.

The Ftp::Server::handleReply() method is called [via Store] from Client code.
Thus, exceptions in handleReply() are handled by the Ftp::Client job. That job
does not have enough information to know whether the client-to-Squid connection
should be closed; the job keeps the connection open. When the reply is nil,
that open connection becomes unusable, leading to more problems.

This patch fixes the Ftp::Server::handleReply() to handle exceptions,
including closing the connections in the case of an exception. It also
adds Must(reply) checks to check for nil HttpReply pointers where the
reply is required. Eventually, Store should start using async calls to
protect jobs waiting for Store updates. Meanwhile, this should help.

This is a Measurement Factory project.