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);