Fix a bug when binding to pointer

[golddranks]

Hi, this one-line PR fixes a problem when binding to a pointer type and the bind fails. At the moment it tries to set to an incompatible type. Here's example code:

	var threshold float64
	var pThreshold *float64
	c.Params.Bind(&pThreshold, "threshold")
	if pThreshold != nil {
		threshold = *pThreshold
	}
	if pThreshold == nil || threshold < 0.0 || threshold > 1.0 {
		_, threshold, err = selectedModel.GetDefaultThreshold()
	}
	if err != nil {
		return c.RenderError(err)
	}

If the param is something that isn't convertible into a float, the return value of the Bind call in binder.go:160 isn't CanAddr(), and it tries to return the value itself, instead of a pointer. But because it is a pointer binder, it is supposed to return a pointer. Returning something else will cause it to crash with "can't set a float64 into a variable of type *float64" Fixing it to return a null pointer fixes the problem.

There is another, API problem with the binder API though. Bindings can fail, but at the moment, there isn't good ways to detect this failure. It would be great if the Bind call would return a boolean that tells whether the binding succeeded or not. As you can see, I'm binding here a float64 type, but I'm using a pointer as a stop-gap resolution to detect when the binding fails.