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.

src/xftextent.c

@@ -123,7 +123,7 @@ XftTextExtents8 (Display	    *dpy,
     FT_UInt	    *glyphs, glyphs_local[NUM_LOCAL];
     int		    i;
 
-    if (len <= 0)
+    if (len < 0)
 	return;
 
     if (len <= NUM_LOCAL)
@@ -154,7 +154,7 @@ XftTextExtents16 (Display	    *dpy,
     FT_UInt	    *glyphs, glyphs_local[NUM_LOCAL];
     int		    i;
 
-    if (len <= 0)
+    if (len < 0)
 	return;
 
     if (len <= NUM_LOCAL)
@@ -185,7 +185,7 @@ XftTextExtents32 (Display	    *dpy,
     FT_UInt	    *glyphs, glyphs_local[NUM_LOCAL];
     int		    i;
 
-    if (len <= 0)
+    if (len < 0)
 	return;
 
     if (len <= NUM_LOCAL)
@@ -219,7 +219,7 @@ XftTextExtentsUtf8 (Display	    *dpy,
     int		    l;
     int		    size;
 
-    if (len <= 0)
+    if (len < 0)
 	return;
 
     i = 0;
@@ -266,7 +266,7 @@ XftTextExtentsUtf16 (Display		*dpy,
     int		    l;
     int		    size;
 
-    if (len <= 0)
+    if (len < 0)
 	return;
 
     i = 0;