libfslvpuwrap: fix tests of return value from IOGetVirtMem

Same as similar patches for imx-vpu, the returned value from
IOGetVirtMem was not properly checked.

[Thomas: add upstream reference to the patch, suggested by Yann
E. Morin.]

Signed-off-by: Gary Bisson <bisson.gary@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>