st/dri: Fix surfaceless gl using contexts with previous bound surfaces

st/dri: Fix surfaceless gl using contexts with previous bound surfaces

ctx->dPriv might be != NULL then draw which is NULL is accessed:

struct dri_drawable *draw = dri_drawable(driDrawPriv);
[..]
if (ctx->dPriv != driDrawPriv) {
      ctx->dPriv = driDrawPriv;
      draw->texture_stamp = driDrawPriv->lastStamp - 1;
}