t-strvec: mark variable-arg helper with LAST_ARG_MUST_BE_NULL

This will let the compiler catch a problem like:

  /* oops, we forgot the NULL */
  check_strvec(&vec, "foo");

rather than triggering undefined behavior at runtime.

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

t/unit-tests/t-strvec.c

@@ -4,6 +4,7 @@
 
 #define check_strvec(vec, ...) \
 	check_strvec_loc(TEST_LOCATION(), vec, __VA_ARGS__)
+LAST_ARG_MUST_BE_NULL
 static void check_strvec_loc(const char *loc, struct strvec *vec, ...)
 {
 	va_list ap;