fixes a memory leak:

==19074== 2 bytes in 1 blocks are definitely lost in loss record 8 of 313
==19074==    at 0x483577F: malloc (vg_replace_malloc.c:299)
==19074==    by 0x1FD83D: fbAllocatePrivates (fballpriv.c:79)
==19074==    by 0x20A666: fbSetupScreen (fbscreen.c:110)
==19074==    by 0x20A666: fbScreenInit (fbscreen.c:300)
==19074==    by 0x1DEA4C: nxagentOpenScreen (Screen.c:1356)
==19074==    by 0x16D7F8: AddScreen (dispatch.c:4257)
==19074==    by 0x1DA0CF: InitOutput (Init.c:397)
==19074==    by 0x14DCC2: main (main.c:280)