netfilter: fix warning in net/netfilter/nf_conntrack_ftp.c

this warning:

  net/netfilter/nf_conntrack_ftp.c: In function 'help':
  net/netfilter/nf_conntrack_ftp.c:360: warning: 'matchoff' may be used uninitialized in this function
  net/netfilter/nf_conntrack_ftp.c:360: warning: 'matchlen' may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between find_pattern(), 'found', 'matchoff' and 'matchlen'.

Annotate it.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>