Fix display when left-fringe/right-fringe display spec is invalid.

Fix display when left-fringe/right-fringe display spec is invalid.

 src/xdisp.c (handle_single_display_spec): Return 1 for left-margin
 and right-margin display specs even if the spec is invalid or we
 are on a TTY, and thus unable to display on the fringes.  That's
 because the text with the property will not be displayed anyway,
 so we need to signal to the caller that this is a "replacing"
 display spec.  This fixes display when the spec is invalid or we
 are on a TTY.