[PATCH] builtin/difftool: intialize some hashmap variables

When running a dir-diff command that produces no diff, variables `wt_modified` and `tmp_modified` are used while uninitialized, causing:

    $ /home/smarchi/src/git/git-difftool --dir-diff master
    free(): invalid pointer
    [1]    334004 IOT instruction (core dumped)  /home/smarchi/src/git/git-difftool --dir-diff master
    $ valgrind --track-origins=yes /home/smarchi/src/git/git-difftool --dir-diff master
    ...
    Invalid free() / delete / delete[] / realloc()
       at 0x48478EF: free (vg_replace_malloc.c:989)
       by 0x422CAC: hashmap_clear_ (hashmap.c:208)
       by 0x283830: run_dir_diff (difftool.c:667)
       by 0x284103: cmd_difftool (difftool.c:801)
       by 0x238E0F: run_builtin (git.c:484)
       by 0x2392B9: handle_builtin (git.c:750)
       by 0x2399BC: cmd_main (git.c:921)
       by 0x356FEF: main (common-main.c:64)
     Address 0x1ffefff180 is on thread 1's stack
     in frame #2, created by run_dir_diff (difftool.c:358)
    ...

If taking any `goto finish` path before these variables are initialized, `hashmap_clear_and_free()` operates on uninitialized data, sometimes causing a crash.

Fix it by zero-initializing these variables, making `hashmap_clear_and_free()` a no-op in that case.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Cc: Junio C Hamano <gitster@pobox.com>
Cc: René Scharfe <l.s.r@web.de>
Cc: Taylor Blau <me@ttaylorr.com>
Cc: Patrick Steinhardt <ps@pks.im>
---
 builtin/difftool.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)