Avoid NULL pointer dereference in xf86TokenToOptinfo if token not found

Avoid NULL pointer dereference in xf86TokenToOptinfo if token not found

Reported by parfait 1.1 code analyzer:

Error: Null pointer dereference (CWE 476)
   Read from null pointer 'p'
        at line 746 of hw/xfree86/common/xf86Option.c in function 'xf86TokenToOptName'.
          Function 'xf86TokenToOptinfo' may return constant 'NULL' at line 721, called at line 745.
          Null pointer introduced at line 721 in function 'xf86TokenToOptinfo'.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>