Handle the unlikely case of reaching the end of an exhaustive enum in

a test to appease GCC.

`error: control reaches end of non-void function [-Werror=return-type]`

PiperOrigin-RevId: 715506532

quiche/http2/adapter/adapter_impl_comparison_test.cc

@@ -44,6 +44,7 @@ class ComparisonTest : public ::quiche::test::QuicheTest {
         options.perspective = p;
         return OgHttp2Adapter::Create(visitor, options);
     }
+    return nullptr;  // Unreachable unless enum is corrupted.
   }
 };