Avoid potentially dangerous signed to unsigned conversion

We make sure to never pass a negative `rlen` as size to memcpy(). See
also <https://bugs.php.net/bug.php?id=73280>.

Patch provided by Emmanuel Law.
static long dynamicTell(struct gdIOCtx *ctx)
{
	dpIOCtx *dctx;

	dctx = (dpIOCtx *)ctx;
	return (dctx->dp->pos);
}

static long dynamicTell(struct gdIOCtx *ctx)
{
	dpIOCtx *dctx;

	dctx = (dpIOCtx *)ctx;
	return (dctx->dp->pos);
}
