Fix panic on zero-value Viper struct SetDefault/Set

[Bahtya]

Problem

Using a zero-value Viper struct (without calling New() or Init()) causes a nil map panic when calling SetDefault() or Set():

var v viper.Viper
v.SetDefault("key", "value") // panic: assignment to entry in nil map

Solution

Add nil-map guards in both SetDefault and Set to initialize the maps on first use:

if v.defaults == nil {
    v.defaults = make(map[string]interface{})
}

Fixes #2111

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

bahtya seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}