t-strvec: use va_end() to match va_start()

t-strvec: use va_end() to match va_start()

Our check_strvec_loc() helper uses a variable argument list. When we
va_start(), we must be sure to va_end() before leaving the function.
This is required by the standard (though the effect of forgetting will
vary between platforms).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>