Enforce a read callback to be specified for decoders.

miniaudio.h

@@ -66237,6 +66237,11 @@ static ma_result ma_decoder__preinit(ma_decoder_read_proc onRead, ma_decoder_see
 
     MA_ZERO_OBJECT(pDecoder);
 
+    /* A read callback must be available. */
+    if (onRead == NULL) {
+        return MA_INVALID_ARGS;
+    }
+
     dataSourceConfig = ma_data_source_config_init();
     dataSourceConfig.vtable = &g_ma_decoder_data_source_vtable;