Check if input_buffer exist

In some really specific cases there is a chance that
one of the buffers is valid, but its pointer is null

src/xenia/apu/xma_context.cc

@@ -346,6 +346,9 @@ void XmaContext::Decode(XMA_CONTEXT_DATA* data) {
            data->input_buffer_read_offset, data->current_buffer,
            current_input_buffer);
 
+  if (!current_input_buffer) {
+    return;
+  }
   size_t input_buffer_0_size =
       data->input_buffer_0_packet_count * kBytesPerPacket;
   size_t input_buffer_1_size =