upload-pack: handle unexpected delim packets

upload-pack: handle unexpected delim packets

When processing the arguments list for a v2 ls-refs or fetch command, we
loop like this:

  while (packet_reader_read(request) != PACKET_READ_FLUSH) {
          const char *arg = request->line;
	  ...handle arg...
  }

to read and handle packets until we see a flush. The hidden assumption
here is that anything except PACKET_READ_FLUSH will give us valid packet
data to read. But that's not true; PACKET_READ_DELIM or PACKET_READ_EOF
will leave packet->line as NULL, and we'll segfault trying to look at
it.

Instead, we should follow the more careful model demonstrated on the
client side (e.g., in process_capabilities_v2): keep looping as long
as we get normal packets, and then make sure that we broke out of the
loop due to a real flush. That fixes the segfault and correctly
diagnoses any unexpected input from the client.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>