Avoid decoding nil pointer in map walker

.changelog/17048.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+Fix an bug where decoding some Config structs with unset pointer fields could fail with `reflect: call of reflect.Value.Type on zero Value`. 
+```

lib/map_walker.go

@@ -113,7 +113,7 @@ func (w *mapWalker) MapElem(m, k, v reflect.Value) error {
 		return nil
 	}
 
-	if inner := v.Elem(); inner.Type() == typMapIfaceIface {
+	if inner := v.Elem(); inner.IsValid() && inner.Type() == typMapIfaceIface {
 		// map[interface{}]interface{}, attempt to weakly decode into string keys
 		var target map[string]interface{}
 		if err := mapstructure.WeakDecode(v.Interface(), &target); err != nil {

lib/map_walker_test.go

@@ -41,6 +41,16 @@ func TestMapWalk(t *testing.T) {
 			},
 			unexpected: true,
 		},
+		// ensure we don't panic from trying to call reflect.Value.Type
+		// on a nil pointer
+		"nil pointer": {
+			input: map[string]interface{}{
+				"foo": nil,
+			},
+			expected: map[string]interface{}{
+				"foo": nil,
+			},
+		},
 		// ensure nested maps get processed correctly
 		"nested": {
 			input: map[string]interface{}{