t_dd_dmatmp: Use addition instead of subtraction in loop bounds

This is used everywhere else in this file because it avoids problems
when count is zero (due to trimming).

No piglit regressions on i915 (G33) or radeon (Radeon 7500).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38109
Reviewed-by: Brian Paul <brianp@vmware.com>
Cc: Marius Predut <marius.predut@intel.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>

src/mesa/tnl_dd/t_dd_dmatmp.h

@@ -623,7 +623,7 @@ static void TAG(render_quads_verts)( struct gl_context *ctx,
 
       INIT(GL_TRIANGLES);
 
-      for (j = 0; j < count-3; j += 4) {
+      for (j = 0; j + 3 < count; j += 4) {
 	 void *tmp = ALLOC_VERTS( 6 );
 	 /* Send v0, v1, v3
 	  */