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/vst24.h

@@ -47,6 +47,14 @@
 [[maybe_unused]] constexpr int effSetSpeakerArrangement = 42;
 [[maybe_unused]] constexpr int effGetSpeakerArrangement = 69;
 
+/**
+ * Used by REAPER for some VST2.4 extensions. Most of the arguments passed to
+ * this will be able to be handled automatically by our `DefaultDataConverter`.
+ * We need one special case because for when they're now using the `data`
+ * argument with a non-pointer value. Found on the same list as above.
+ */
+[[maybe_unused]] constexpr int effVendorSpecific = 50;
+
 /**
  * Used by hosts to query the length of reverb tails (equivalent to
  * `IAudioProcessor::getTailSamples`). Found on the same list as above.