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.