fix: return error instead of panicking when MapTo receives a nil pointer

[Yanhu007]

Fixes #369

Problem

When MapTo is called with a pointer to a nil struct pointer, the code dereferences once and then operates on the nil value, causing a panic:

var cfg *Config // nil
f.MapTo(&cfg)  // panics!

Fix

Add a loop after the initial pointer unwrap to handle nested pointers. If a nil pointer is encountered, return a clear error instead of panicking:

for val.Kind() == reflect.Ptr {
    if val.IsNil() {
        return errors.New("cannot map to a nil pointer")
    }
    typ = typ.Elem()
    val = val.Elem()
}

All existing tests pass.