Use sizeof() for strncpy's dest, instead of a dissociated constant

This small DRY change also fixes a GCC 8 warning:

  csv.h:78:48: warning: ‘char* strncpy(char*, const char*, size_t)’ output may be truncated copying 255 bytes from a string of length 255 [-Wstringop-truncation]

GCC is unable to determine that error::max_file_name_length is the
file_name - using sizeof() helps here.

Author: pboettch

csv.h

@@ -70,13 +70,13 @@ namespace io{
 
                 struct with_file_name{
                         with_file_name(){
-                                std::memset(file_name, 0, max_file_name_length+1);
+                                std::memset(file_name, 0, sizeof(file_name));
                         }
 
                         void set_file_name(const char*file_name){
                                 if(file_name != nullptr){
-                                        strncpy(this->file_name, file_name, error::max_file_name_length);
-                                        this->file_name[error::max_file_name_length] = '\0';
+                                        strncpy(this->file_name, file_name, sizeof(this->file_name));
+                                        this->file_name[sizeof(this->file_name)-1] = '\0';
                                 }else{
                                         this->file_name[0] = '\0';
                                 }
@@ -422,8 +422,8 @@ namespace io{
 
                 void set_file_name(const char*file_name){
                         if(file_name != nullptr){
-                                strncpy(this->file_name, file_name, error::max_file_name_length);
-                                this->file_name[error::max_file_name_length] = '\0';
+                                strncpy(this->file_name, file_name, sizeof(this->file_name));
+                                this->file_name[sizeof(this->file_name)-1] = '\0';
                         }else{
                                 this->file_name[0] = '\0';
                         }