Ensure button-get works in any buffer

* lisp/button.el (button-get): Previously we assumed that button-get
  was called in the buffer containing the button.  In other buffers,
  button-get always returned nil.  Fix this by passing the relevant
  buffer from the marker.

lisp/button.el

@@ -194,6 +194,8 @@ changes to a supertype are not reflected in its subtypes)."
 	((button--area-button-p button)
 	 (get-text-property (cdr button)
 			    prop (button--area-button-string button)))
+	((markerp button)
+	 (get-text-property button prop (marker-buffer button)))
 	(t ; Must be a text-property button.
 	 (get-text-property button prop))))