Fix buffer overflow in 15-tuple ToJSON instance

Without this patch:

    > encode (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)
    "[1,2,3,4,5,6,7,8,9,10,11,12,13,14]"

With this patch:

    > encode (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)
    "[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]"

This happens because the code writes 15 elements to the vector, but only allocates enough space for 14.