gpg: Clarify a call of ask_for_detached_datafile.

* g10/mainproc.c (proc_tree): Call ask_for_detached_datafile
with MD2=NULL.

--

Here, c->mfx.md2 is always NULL, in fact.  But, text-wise
(when searching the use of "mfx.md2"), before the change, it used
mfx.md2, which is irrelevant in the handling of PKT_ONEPASS_SIG.

Note that: One-Pass Signature is not available in PGP2.

This fix removes (text-wise) unmatch of the calls of functions
hash_datafile_by_fd hash_datafiles, and ask_for_detached_datafile.

Fixes-commit: 88a916cdd4
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

g10/mainproc.c

@@ -2544,7 +2544,7 @@ proc_tree (CTX c, kbnode_t node)
 	    }
           else
             {
-              rc = ask_for_detached_datafile (c->mfx.md, c->mfx.md2,
+              rc = ask_for_detached_datafile (c->mfx.md, NULL,
                                               iobuf_get_real_fname (c->iobuf),
                                               use_textmode);
 	    }