In bind_inputs() set VB->EdgeFlag to NULL if it's not needed.

Otherwise, the clip/interp code was finding VB->EdgeFlag to be non-null and
reading/writing it when the memory may have been freed earlier in free_space().
This fixes several VTK segfaults/failures reported by Brad King @ Kitware.

src/mesa/tnl/t_draw.c

@@ -251,7 +251,10 @@ static void bind_inputs( GLcontext *ctx,
 					   VB->AttribPtr[_TNL_ATTRIB_EDGEFLAG],
 					   VB->Count );
    }
-
+   else {
+      /* the data previously pointed to by EdgeFlag may have been freed */
+      VB->EdgeFlag = NULL;
+   }
 }