[tree-ssa-loop-ch] Add missing NULL test for dump_file

[tree-ssa-loop-ch] Add missing NULL test for dump_file

If we change gimple_can_duplicate_bb_p to return false instead of true, we run
into a segfault in ch_base::copy_headers due to using dump_file while it's
NULL:
...
      if (!gimple_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs,
                                        true))
       {
         fprintf (dump_file, "Duplication failed.\n");
         continue;
       }
...

Fix this by adding the missing dump_file != NULL test.

Tested by rebuilding lto1 and rerunning the failing test-case.

gcc/ChangeLog:

2020-10-07  Tom de Vries  <tdevries@suse.de>

	* tree-ssa-loop-ch.c (ch_base::copy_headers): Add missing NULL test
	for dump_file.