file: Fix double-free along error path

  $ mkdir j; nbdkit -U - -fv -r file dir=j \
                    --run 'nbdsh --opt-mode -u "$uri" -c "h.opt_info()"'

crashes at:

  libguestfs#7  0x00007f244789f6ae in __GI___libc_free (mem=0x7f2440000ee0) at malloc.c:3386
  libguestfs#8  0x00007f2447f1a5da in file_open (readonly=1) at /home/rjones/d/nbdkit/plugins/file/file.c:615

Reported-by: Eric Blake

Author: rwmjones

plugins/file/file.c

@@ -504,7 +504,6 @@ open_file_by_name (struct handle *h, int readonly, int dfd, const char *file)
   }
   if (h->fd == -1) {
     nbdkit_error ("open: %s: %m", file);
-    free (h);
     return -1;
   }