[APU] Skip audio header when there is no valid input

Thanks Cancerous1/Randprint for initial reseach in this topic

src/xenia/apu/xma_context.cc

@@ -301,7 +301,9 @@ void XmaContext::Decode(XMA_CONTEXT_DATA* data) {
 
   // No available data.
   if (!data->input_buffer_0_valid && !data->input_buffer_1_valid) {
-    data->output_buffer_valid = 0;
+    // 4156081D checks specifically for offset 0x20 when both input buffers
+    // are invalid.
+    data->input_buffer_read_offset = kBitsPerHeader;
     return;
   }