Fix null data_

Author: bblanchon

src/ArduinoJson/Variant/VariantImpl.hpp

@@ -19,7 +19,7 @@ inline void VariantImpl::setRawString(SerializedValue<T> value) {
 }
 
 inline bool VariantImpl::setLinkedString(const char* s) {
-  ARDUINOJSON_ASSERT(type() == VariantType::Null);  // must call clear() first
+  ARDUINOJSON_ASSERT(isNull());  // must call clear() first
   ARDUINOJSON_ASSERT(s);
 
   auto slotId = resources_->saveStaticString(s);
@@ -33,7 +33,7 @@ inline bool VariantImpl::setLinkedString(const char* s) {
 
 template <typename TAdaptedString>
 inline bool VariantImpl::setString(TAdaptedString value) {
-  ARDUINOJSON_ASSERT(type() == VariantType::Null);  // must call clear() first
+  ARDUINOJSON_ASSERT(isNull());  // must call clear() first
 
   if (!data_)
     return false;
@@ -90,7 +90,10 @@ inline const char* VariantImpl::asLinkedString() const {
 
 template <typename T>
 enable_if_t<sizeof(T) == 8, bool> VariantImpl::setFloat(T value) {
-  ARDUINOJSON_ASSERT(type() == VariantType::Null);  // must call clear() first
+  ARDUINOJSON_ASSERT(isNull());  // must call clear() first
+
+  if (!data_)
+    return false;
 
   float valueAsFloat = static_cast<float>(value);
 
@@ -115,7 +118,10 @@ enable_if_t<sizeof(T) == 8, bool> VariantImpl::setFloat(T value) {
 
 template <typename T>
 enable_if_t<is_signed<T>::value, bool> VariantImpl::setInteger(T value) {
-  ARDUINOJSON_ASSERT(type() == VariantType::Null);  // must call clear() first
+  ARDUINOJSON_ASSERT(isNull());  // must call clear() first
+
+  if (!data_)
+    return false;
 
   if (canConvertNumber<int32_t>(value)) {
     data_->type = VariantType::Int32;
@@ -136,7 +142,10 @@ enable_if_t<is_signed<T>::value, bool> VariantImpl::setInteger(T value) {
 
 template <typename T>
 enable_if_t<is_unsigned<T>::value, bool> VariantImpl::setInteger(T value) {
-  ARDUINOJSON_ASSERT(type() == VariantType::Null);  // must call clear() first
+  ARDUINOJSON_ASSERT(isNull());  // must call clear() first
+
+  if (!data_)
+    return false;
 
   if (canConvertNumber<uint32_t>(value)) {
     data_->type = VariantType::Uint32;