show-branch: fix SEGFAULT when --current and --reflog together

If run `show-branch` with `--current` and `--reflog` simultaneously, a
SEGFAULT appears.

The bug is that we read over the end of the `reflog_msg` array after
having `append_one_rev()` for the current branch without supplying a
convenient message to it.

It seems that it has been introduced in:
Commit 1aa68d6 (show-branch: --current includes the current branch.,
2006-01-11)

Signed-off-by: Gregory DAVID <[email protected]>
Thanks-to: Ævar Arnfjörð Bjarmason <[email protected]>

Author: p1-gdd

builtin/show-branch.c

@@ -813,6 +813,26 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 		}
 		if (!has_head) {
 			const char *name = head;
+			struct object_id oid;
+			char *ref;
+			unsigned int flags = 0;
+			char *log_msg;
+			char *end_log_msg;
+			timestamp_t timestamp;
+			int tz;
+
+			if (!dwim_ref(*av, strlen(*av), &oid, &ref, 0))
+				die(_("no such ref %s"), *av);
+			if(read_ref_at(get_main_ref_store(the_repository),
+					ref, flags, 0, i, &oid, &log_msg,
+					&timestamp, &tz, NULL)) {
+				end_log_msg = strchr(log_msg, '\n');
+				if (end_log_msg)
+					*end_log_msg = '\0';
+			}
+			if(log_msg == 0 || *log_msg == '\0')
+				log_msg = xstrfmt("(none)");
+			reflog_msg[ref_name_cnt] = xstrfmt("(%s) (current) %s", show_date(timestamp, tz, DATE_MODE(RELATIVE)), log_msg);
 			skip_prefix(name, "refs/heads/", &name);
 			append_one_rev(name);
 		}