Header: allocate memory for terminating entry

The names array is terminated by a NULL entry, thus allocate space for
one more than entries.

Fixes: #844

==6708==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6060000045b8 at pc 0x000000589ee1 bp 0x7ffcd1dee220 sp 0x7ffcd1dee218
READ of size 8 at 0x6060000045b8 thread T0
    #0 0x589ee0 in String_freeArray ./XUtils.c:157:23
    #1 0x56c9af in Settings_delete ./Settings.c:31:7
    #2 0x4ee44b in CommandLine_run ./CommandLine.c:395:4
    #3 0x4d6fb2 in main ./htop.c:15:11
    #4 0x7ff3b8154e49 in __libc_start_main csu/../csu/libc-start.c:314:16
    #5 0x428aa9 in _start (./htop+0x428aa9)

0x6060000045b8 is located 0 bytes to the right of 56-byte region [0x606000004580,0x6060000045b8)
allocated by thread T0 here:
    #0 0x4a53f2 in __interceptor_calloc (./htop+0x4a53f2)
    #1 0x5890ba in xCalloc ./XUtils.c:55:17
    #2 0x50a044 in Header_writeBackToSettings ./Header.c:148:34
    #3 0x4de861 in Action_runSetup ./Action.c:91:7
    #4 0x4de861 in actionSetup ./Action.c:386:4
    #5 0x515caf in MainPanel_eventHandler ./MainPanel.c:106:19
    #6 0x56a5c1 in ScreenManager_run ./ScreenManager.c:235:19
    #7 0x4ee13b in CommandLine_run ./CommandLine.c:364:4
    #8 0x4d6fb2 in main ./htop.c:15:11
    #9 0x7ff3b8154e49 in __libc_start_main csu/../csu/libc-start.c:314:16

Header.c

@@ -145,7 +145,7 @@ void Header_writeBackToSettings(const Header* this) {
       const Vector* vec = this->columns[col];
       int len = Vector_size(vec);
 
-      colSettings->names = len ? xCalloc(len, sizeof(char*)) : NULL;
+      colSettings->names = len ? xCalloc(len + 1, sizeof(char*)) : NULL;
       colSettings->modes = len ? xCalloc(len, sizeof(int)) : NULL;
       colSettings->len = len;