avoid future chance of using uninitialized memory

* common/iobuf.c: (iobuf_open): initialize len

--

In iobuf_open, IOBUFCTRL_DESC and IOBUFCTRL_INIT commands are invoked
(via file_filter()) on fcx, passing in a pointer to an uninitialized
len.

With these two commands, file_filter doesn't actually do anything with
the value of len, so there's no actual risk of use of uninitialized
memory in the code as it stands.

However, some static analysis tools might flag this situation with a
warning, and initializing the value doesn't hurt anything, so i think
this trivial cleanup is warranted.

Debian-Bug-Id: 773469

common/iobuf.c

@@ -1301,7 +1301,7 @@ iobuf_open (const char *fname)
   iobuf_t a;
   gnupg_fd_t fp;
   file_filter_ctx_t *fcx;
-  size_t len;
+  size_t len = 0;
   int print_only = 0;
   int fd;