Fix current-bidi-paragraph-direction wrt window pointer.

Fix current-bidi-paragraph-direction wrt window pointer.

 src/xdisp.c (compute_display_string_pos): When W is NULL, use the current
 buffer as the object to pass to get-char-property.
 (Fcurrent_bidi_paragraph_direction): Assign NULL to the window
 pointer member of the bidi iterator, since no window is pertinent
 to this function.