When processing a truncated format string, uc_printf_common() - which is
used by `sprintf()` and `printf()` in ucode - appended trailing garbage
to the resulting string.

-- Expect stdout --
[ 37, null ]
-- End --

-- Testcase --
{%
	let s = sprintf("%");
	print([ ord(s, 0), ord(s, 1) ], "\n");
%}
-- End --