Fix the canProcessSampleSize() cache

This is what you get for carelessly copying and pasting.

src/plugin/bridges/vst3-impls/plugin-proxy.cpp

@@ -157,7 +157,7 @@ Vst3PluginProxyImpl::canProcessSampleSize(int32 symbolicSampleSize) {
                     true);
             }
 
-            return *function_result_cache.parameter_count;
+            return it->second;
         }
     }