mm/sparse.c: check the return value of sparse_index_alloc()

Since sparse_index_alloc() can return NULL on memory allocation failure,
we must deal with the failure condition when calling it.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>