init: handle empty "template" parameter

If a user passes "--template=", then our template parameter
is blank. Unfortunately, copy_templates() assumes it has at
least one character, and does all sorts of bad things like
reading from template[-1] and then proceeding to link all of
'/' into the .git directory.

This patch just checks for that condition in copy_templates
and aborts. As a side effect, this means that --template=
now has the meaning "don't copy any templates."

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-init-db.c

@@ -127,6 +127,8 @@ static void copy_templates(const char *template_dir)
 			template_dir = strbuf_detach(&d, NULL);
 		}
 	}
+	if (!template_dir[0])
+		return;
 	strcpy(template_path, template_dir);
 	template_len = strlen(template_path);
 	if (template_path[template_len-1] != '/') {