Commit 06a3c0ab in libXft 2.3.5 added length checks of the form if (len <= 0) return; to various Xft functions. However, while rendering an empty string is equivalent to doing nothing, asking for the extents of an empty string isn't -- it still needs to fill in the extents structure. This broke text rendering in some applications (e.g. xpdf's Motif GUI).
Check for len < 0 in XftTextExtents* instead.
(If similar changes have been made elsewhere, it'd be worth checking they don't have the same problem...)