mesa: modified _mesa_align_free() to accept NULL pointer

So that it acts like ordinary free().  This lets us remove a bunch of
if statements where the function is called.

v2:
- Avoiding compile error on MSVC and possible warnings on other compilers.
- Added comment regards passing NULL pointer being safe.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/tnl/t_vertex.c

@@ -546,10 +546,8 @@ void _tnl_free_vertices( struct gl_context *ctx )
       struct tnl_clipspace *vtx = GET_VERTEX_STATE(ctx);
       struct tnl_clipspace_fastpath *fp, *tmp;
 
-      if (vtx->vertex_buf) {
-         _mesa_align_free(vtx->vertex_buf);
-         vtx->vertex_buf = NULL;
-      }
+      _mesa_align_free(vtx->vertex_buf);
+      vtx->vertex_buf = NULL;
 
       for (fp = vtx->fastpath ; fp ; fp = tmp) {
          tmp = fp->next;