reflect: don't panic on delete from nil map.

Fixes #8051

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/95560046
diff --git a/src/pkg/reflect/all_test.go b/src/pkg/reflect/all_test.go
index 9c5eb4e..e994901 100644
--- a/src/pkg/reflect/all_test.go
+++ b/src/pkg/reflect/all_test.go
@@ -993,6 +993,9 @@
 	if x.Kind() != Invalid {
 		t.Errorf("mbig.MapIndex(\"hello\") for nil map = %v, want Invalid Value", x)
 	}
+
+	// Test that deletes from a nil map succeed.
+	mv.SetMapIndex(ValueOf("hi"), Value{})
 }
 
 func TestChan(t *testing.T) {