[PATCH 2/2] ll_union_merge(): pass name labels to ll_xdl_merge()

Since cd1d61c44f (make union merge an xdl merge favor, 2010-03-01), we pass NULL to ll_xdl_merge() for the "name" labels of the ancestor, ours and theirs buffers. We usually use these for annotating conflict markers left in a file. For a union merge, these shouldn't matter; the point of it is that we'd never leave conflict markers in the first place.

But there is one code path where we may dereference them: if the file contents appear to be binary, ll_binary_merge() will give up and pass them to warning() to generate a message for the user (that was true even when cd1d61c44f was written, though the warning was in ll_xdl_merge() back then).

That can result in a segfault, though on many systems (including glibc), the printf routines will helpfully just say "(null)" instead. We can extend our binary-union test in t6406 to check stderr, which catches the problem on all systems.

This also fixes a warning from "gcc -O3". Unlike lower optimization levels, it inlines enough to see that the NULL can make it to warning() and complains:

  In function ‘ll_binary_merge’,
      inlined from ‘ll_xdl_merge’ at ll-merge.c:115:10,
      inlined from ‘ll_union_merge’ at ll-merge.c:151:9:
  ll-merge.c:74:4: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
     74 |    warning("Cannot merge binary files: %s (%s vs. %s)",
        |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     75 |     path, name1, name2);
        |     ~~~~~~~~~~~~~~~~~~~

Signed-off-by: Jeff King <peff@peff.net>
---
 ll-merge.c            | 2 +-
 t/t6406-merge-attr.sh | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)