Fix bug with accessing memzero'd X509StoreContext in tests

As of https://boringssl-review.googlesource.com/c/boringssl/+/64141,
X509_STORE_CTX_cleanup will zero the memory allocated to the
X509_STORE_CTX. Because X509StoreContextRef::init invokes
X509_STORE_CTX_cleanup once the with_context closure has finished,
calling X509StoreContextRef::verify_result (or any API really) is going
to be invalid because memory has been zerod out. This is a pretty big
footgun, so maybe we should consider screaming a bit louder for this
case.

boring/src/x509/tests/trusted_first.rs

@@ -93,12 +93,12 @@ fn verify(
 
     let mut store_ctx = X509StoreContext::new().unwrap();
 
-    let _ = store_ctx.init(&trusted, cert, &untrusted, |ctx| {
-        configure(ctx.verify_param_mut());
-        ctx.verify_cert().unwrap();
-
-        Ok(())
-    });
-
-    store_ctx.verify_result()
+    store_ctx
+        .init(&trusted, cert, &untrusted, |ctx| {
+            configure(ctx.verify_param_mut());
+            ctx.verify_cert().unwrap();
+
+            Ok(ctx.verify_result())
+        })
+        .expect("failed to obtain X509VerifyResult")
 }