[api-fuzzer] Fix inproc channels (#35538)

Closes #35538

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35538 from ctiller:fff2 52b69968a0
PiperOrigin-RevId: 597848114

test/core/end2end/fuzzers/api_fuzzer.cc

@@ -434,6 +434,7 @@ ApiFuzzer::Result ApiFuzzer::CreateChannel(
   ChannelArgs args = testing::CreateChannelArgsFromFuzzingConfiguration(
       create_channel.channel_args(), fuzzing_env);
   if (create_channel.inproc()) {
+    if (server_ == nullptr) return Result::kFailed;
     channel_ = grpc_inproc_channel_create(server_, args.ToC().get(), nullptr);
   } else {
     grpc_channel_credentials* creds =

test/core/end2end/fuzzers/api_fuzzer_corpus/5611761370202112

@@ -0,0 +1,5 @@
+actions {
+  create_channel {
+    inproc: true
+  }
+}