history: initialize rev_info in cmd_history_reword()

git history reword expects a single valid revision argument and errors
out if it doesn't get it.  In that case the struct rev_info passed to
release_revisions() for cleanup is still uninitialized, which can result
in attempts to free(3) random pointers.  Avoid that by initializing the
structure.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/history.c

@@ -425,7 +425,7 @@ static int cmd_history_reword(int argc,
 	};
 	struct strbuf reflog_msg = STRBUF_INIT;
 	struct commit *original, *rewritten;
-	struct rev_info revs;
+	struct rev_info revs = { 0 };
 	int ret;
 
 	argc = parse_options(argc, argv, prefix, options, usage, 0);