check malloc return value for NULL and update docs

`sfFtpDirectoryResponse_getDirectory` can fail and return NULL becuase 'strdup' returns null on failure but `sfFtpDirectoryResponse_getDirectoryUnicode` didn't and just had UB due not checking return value of malloc.

typing NULL in the docs made me age 30 years btw

include/CSFML/Network/Ftp.h

@@ -225,7 +225,7 @@ CSFML_NETWORK_API const char* sfFtpDirectoryResponse_getMessage(const sfFtpDirec
 ///
 /// \param ftpDirectoryResponse Ftp directory response
 ///
-/// \return Directory name
+/// \return Directory name or NULL if it failed
 ///
 ////////////////////////////////////////////////////////////
 CSFML_NETWORK_API const char* sfFtpDirectoryResponse_getDirectory(const sfFtpDirectoryResponse* ftpDirectoryResponse);
@@ -239,7 +239,7 @@ CSFML_NETWORK_API const char* sfFtpDirectoryResponse_getDirectory(const sfFtpDir
 ///
 /// \param ftpDirectoryResponse Ftp directory response
 ///
-/// \return Directory name
+/// \return Directory name or NULL if it failed
 ///
 ////////////////////////////////////////////////////////////
 CSFML_NETWORK_API const sfChar32* sfFtpDirectoryResponse_getDirectoryUnicode(const sfFtpDirectoryResponse* ftpDirectoryResponse);

src/CSFML/Network/Ftp.cpp

@@ -51,6 +51,8 @@ static_assert(alignof(sfChar32) == alignof(char32_t));
 {
     const std::size_t byteCount = sizeof(sfChar32) * str.getSize();
     auto*             utf32     = static_cast<sfChar32*>(std::malloc(byteCount + sizeof(sfChar32)));
+    if (!utf32)
+        return nullptr;
     std::memcpy(utf32, str.getData(), byteCount);
     utf32[str.getSize()] = 0;