liblzma: index_decoder: Fix missing initializations on LZMA_PROG_ERROR

liblzma: index_decoder: Fix missing initializations on LZMA_PROG_ERROR

If the arguments to lzma_index_decoder() or lzma_index_buffer_decode()
were such that LZMA_PROG_ERROR was returned, the lzma_index **i
argument wasn't touched even though the API docs say that *i = NULL
is done if an error occurs. This obviously won't be done even now
if i == NULL but otherwise it is best to do it due to the wording
in the API docs.

In practice this matters very little: The problem can occur only
if the functions are called with invalid arguments, that is,
the calling application must already have a bug.