DataBuf::c_data() returns nullptr when empty + avoid duplication

After the previous commit, I realized that std::vector::data() also
returns nullptr when the vector is empty. So I decided to emulate this
behavior in DataBuf::c_data().

Anyways, the changes done in the previous commit are valid and allow us
to avoid some processing when the DataBuf is empty.

include/exiv2/types.hpp

@@ -201,7 +201,7 @@ struct EXIV2API DataBuf {
   int cmpBytes(size_t offset, const void* buf, size_t bufsize) const;
 
   //! Returns a data pointer.
-  byte* data(size_t offset = 0);
+  [[nodiscard]] byte* data(size_t offset = 0);
 
   //! Returns a (read-only) data pointer.
   [[nodiscard]] const byte* c_data(size_t offset = 0) const;

src/types.cpp

@@ -172,21 +172,14 @@ int Exiv2::DataBuf::cmpBytes(size_t offset, const void* buf, size_t bufsize) con
 }
 
 byte* Exiv2::DataBuf::data(size_t offset) {
-  if (offset >= pData_.size()) {
-    throw std::out_of_range("Overflow in Exiv2::DataBuf::c_data");
-  }
-  if (pData_.empty() || pData_.size() == offset) {
-    return nullptr;
-  }
-  return &pData_[offset];
+  return const_cast<byte*>(c_data(offset));
 }
 
 const byte* Exiv2::DataBuf::c_data(size_t offset) const {
-  if (offset >= pData_.size()) {
-    throw std::out_of_range("Overflow in Exiv2::DataBuf::c_data");
-  }
-  if (pData_.empty() || pData_.size() == offset) {
+  if (pData_.empty()) {
     return nullptr;
+  } else if (offset >= pData_.size()) {
+    throw std::out_of_range("Overflow in Exiv2::DataBuf::c_data");
   }
   return &pData_[offset];
 }

unitTests/test_types.cpp

@@ -48,7 +48,8 @@ TEST(DataBuf, canBeConstructedFromExistingData) {
 TEST(DataBuf, tryingToAccessTooFarElementThrows) {
   const std::array<byte, 4> data {'h', 'o', 'l', 'a'};
   DataBuf instance(data.data(), data.size());
-  ASSERT_THROW(instance.data(4), std::out_of_range);
+  ASSERT_THROW([[maybe_unused]] auto d = instance.data(4), std::out_of_range);
+  ASSERT_THROW([[maybe_unused]] auto d = instance.c_data(4), std::out_of_range);
 }
 
 // Test methods like DataBuf::read_uint32 and DataBuf::write_uint32.