spapr_nvram: Check return value from blk_getlength()

The blk_getlength() function can return an error value if the
image size cannot be determined. Check for this rather than
ploughing on and trying to g_malloc0() a negative number.
(Spotted by Coverity, CID 1288484.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
