Fix length check in XftTextExtents*.
Commit 06a3c0ab 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.