input_plugin: add method check()

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

src/input_stream.h

@@ -90,6 +90,15 @@ gcc_nonnull(1)
 void
 input_stream_close(struct input_stream *is);
 
+/**
+ * Check for errors that may have occurred in the I/O thread.
+ *
+ * @return false on error
+ */
+gcc_nonnull(1)
+bool
+input_stream_check(struct input_stream *is, GError **error_r);
+
 /**
  * Update the public attributes.  Call before accessing attributes
  * such as "ready" or "offset".