buffer: Fix regression introduced by 6c7eaa and c306a6

Buffer (un)packing was broken on compilers that are not
gcc-compatible since the checks for an argument count of
-1 have been removed from ssh_buffer_(un)pack(). This
fix no longer uses GCC extensions for the __VA_NARG__
macro, but only plain C99.
Note: The macro can no longer count empty argument lists
(results in compile error) which was not needed anyway.

Signed-off-by: Tilo Eckert <tilo.eckert@flam.de>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

tests/unittests/torture_threads_buffer.c

@@ -496,10 +496,8 @@ static void *thread_buffer_pack_badformat(void *threadid)
 
     /* with additional format */
     rc = ssh_buffer_pack(buffer, "bb", b);
-#ifdef HAVE_GCC_NARG_MACRO
-    /* We can only detect errors if we have support for NARG macros */
+    /* check that we detect the missing parameter */
     assert_int_equal(rc, SSH_ERROR);
-#endif
 
     /* unpack with missing format */
     ssh_buffer_reinit(buffer);