Check for nil before binding pflag(s)

* When passing nil to BindPFlag or BindPFlags, the value is set to
  a struct and passed as an interface. That struct never checks for the
  flag(set) being nil. Thus, it makes sense to check before it's set to
  the struct.
* fixes #422

viper.go

@@ -811,6 +811,9 @@ func (v *Viper) UnmarshalExact(rawVal interface{}) error {
 // name as the config key.
 func BindPFlags(flags *pflag.FlagSet) error { return v.BindPFlags(flags) }
 func (v *Viper) BindPFlags(flags *pflag.FlagSet) error {
+	if flags == nil {
+		return fmt.Errorf("FlagSet cannot be nil")
+	}
 	return v.BindFlagValues(pflagValueSet{flags})
 }
 
@@ -822,6 +825,9 @@ func (v *Viper) BindPFlags(flags *pflag.FlagSet) error {
 //
 func BindPFlag(key string, flag *pflag.Flag) error { return v.BindPFlag(key, flag) }
 func (v *Viper) BindPFlag(key string, flag *pflag.Flag) error {
+	if flag == nil {
+		return fmt.Errorf("flag for %q is nil", key)
+	}
 	return v.BindFlagValue(key, pflagValue{flag})
 }
 

viper_test.go

@@ -577,6 +577,14 @@ func TestBindPFlagsStringSlice(t *testing.T) {
 	}
 }
 
+func TestBindPFlagsNil(t *testing.T) {
+	v := New()
+	err := v.BindPFlags(nil)
+	if err == nil {
+		t.Fatalf("expected error when passing nil to BindPFlags")
+	}
+}
+
 func TestBindPFlag(t *testing.T) {
 	var testString = "testing"
 	var testValue = newStringValue(testString, &testString)
@@ -598,6 +606,14 @@ func TestBindPFlag(t *testing.T) {
 
 }
 
+func TestBindPFlagNil(t *testing.T) {
+	v := New()
+	err := v.BindPFlag("any", nil)
+	if err == nil {
+		t.Fatalf("expected error when passing nil to BindPFlag")
+	}
+}
+
 func TestBoundCaseSensitivity(t *testing.T) {
 	assert.Equal(t, "brown", Get("eyes"))