zram: avoid null access when fail to alloc meta

(cherry pick from commit db5d711e2db776f18219b033e5dc4fb7e4264dd7)

zram_meta_alloc could fail so caller should check it.  Otherwise, your
system will hang.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Bug: 24810447
Change-Id: Iba3a0fb8b26c5d9964b7716bfee1844a3896088d