t/unit-tests/clar: fix -Wmaybe-uninitialized with -Og

When building with -Og on gcc 15.1.1, the build produces a warning. In
practice, though, this cannot be hit because `exact` acts as a guard and
that variable can only be set after `matchlen` is already initialized

Assign a default value to `matchlen` so that the warning is silenced.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/clar/clar.c

@@ -350,7 +350,7 @@ static void
 clar_run_suite(const struct clar_suite *suite, const char *filter)
 {
 	const struct clar_func *test = suite->tests;
-	size_t i, matchlen;
+	size_t i, matchlen = 0;
 	struct clar_report *report;
 	int exact = 0;