Fix segfault in REAPER due to new vendor extension

This `effVendorSpecific` call would pass a non-zero non-pointer value to
the pointer argument, which would then of course result in segfaults.

src/common/logging/vst2.cpp

@@ -156,6 +156,9 @@ std::optional<std::string> opcode_to_string(bool is_dispatch, int opcode) {
             case effGetSpeakerArrangement:
                 return "effGetSpeakerArrangement";
                 break;
+            case effVendorSpecific:
+                return "effVendorSpecific";
+                break;
             case effGetTailSize:
                 return "effGetTailSize";
                 break;