Clarify SD longname string size (#14148)

Marlin/src/sd/cardreader.cpp

         char **CardReader::sortshort, **CardReader::sortnames;
       #else
         char CardReader::sortshort[SDSORT_LIMIT][FILENAME_LENGTH];
-        char CardReader::sortnames[SDSORT_LIMIT][SORTED_LONGNAME_MAXLEN];
+        char CardReader::sortnames[SDSORT_LIMIT][SORTED_LONGNAME_STORAGE];
       #endif
     #elif DISABLED(SDSORT_USES_STACK)
-      char CardReader::sortnames[SDSORT_LIMIT][SORTED_LONGNAME_MAXLEN];
+      char CardReader::sortnames[SDSORT_LIMIT][SORTED_LONGNAME_STORAGE];
     #endif
 
     #if HAS_FOLDER_SORTING
       #endif
     #else
       // Copy filenames into the static array
-      #if SORTED_LONGNAME_MAXLEN != LONG_FILENAME_LENGTH
-        #define SET_SORTNAME(I) do{ strncpy(sortnames[I], longest_filename(), SORTED_LONGNAME_MAXLEN); \
-                                    sortnames[I][SORTED_LONGNAME_MAXLEN] = '\0'; }while(0)
+      #define _SET_SORTNAME(I) strncpy(sortnames[I], longest_filename(), SORTED_LONGNAME_MAXLEN)
+      #if SORTED_LONGNAME_MAXLEN == LONG_FILENAME_LENGTH
+        // Short name sorting always use LONG_FILENAME_LENGTH with no trailing nul
+        #define SET_SORTNAME(I) _SET_SORTNAME(I)
       #else
-        #define SET_SORTNAME(I) strncpy(sortnames[I], longest_filename(), SORTED_LONGNAME_MAXLEN)
+        // Copy multiple name blocks. Add a nul for the longest case.
+        #define SET_SORTNAME(I) do{ _SET_SORTNAME(I); sortnames[I][SORTED_LONGNAME_MAXLEN] = '\0'; }while(0)
       #endif
       #if ENABLED(SDSORT_CACHE_NAMES)
         #define SET_SORTSHORT(I) strcpy(sortshort[I], filename)
             sortnames = new char*[fileCnt];
           #endif
         #elif ENABLED(SDSORT_USES_STACK)
-          char sortnames[fileCnt][SORTED_LONGNAME_MAXLEN];
+          char sortnames[fileCnt][SORTED_LONGNAME_STORAGE];
         #endif
 
         // Folder sorting needs 1 bit per entry for flags.

Marlin/src/sd/cardreader.h

     #endif
 
     #if BOTH(SDSORT_USES_RAM, SDSORT_CACHE_NAMES) && DISABLED(SDSORT_DYNAMIC_RAM)
-      #define SORTED_LONGNAME_MAXLEN ((SDSORT_CACHE_VFATS) * (FILENAME_LENGTH) + 1)
+      #define SORTED_LONGNAME_MAXLEN (SDSORT_CACHE_VFATS) * (FILENAME_LENGTH)
+      #define SORTED_LONGNAME_STORAGE (SORTED_LONGNAME_MAXLEN + 1)
     #else
       #define SORTED_LONGNAME_MAXLEN LONG_FILENAME_LENGTH
+      #define SORTED_LONGNAME_STORAGE SORTED_LONGNAME_MAXLEN
     #endif
 
     // Cache filenames to speed up SD menus.
           static char **sortshort, **sortnames;
         #else
           static char sortshort[SDSORT_LIMIT][FILENAME_LENGTH];
-          static char sortnames[SDSORT_LIMIT][SORTED_LONGNAME_MAXLEN];
         #endif
-      #elif DISABLED(SDSORT_USES_STACK)
-        static char sortnames[SDSORT_LIMIT][SORTED_LONGNAME_MAXLEN];
+      #endif
+
+      #if (ENABLED(SDSORT_CACHE_NAMES) && DISABLED(SDSORT_DYNAMIC_RAM)) || NONE(SDSORT_CACHE_NAMES, SDSORT_USES_STACK)
+        static char sortnames[SDSORT_LIMIT][SORTED_LONGNAME_STORAGE];
       #endif
 
       // Folder sorting uses an isDir array when caching items.