mempool: fix crash when handler not found

mempool: fix crash when handler not found

In case the stack or ring mempool handler are compiled as shared
library and not linked in with test binary, segfault is reported.
This is because return value of rte_mempool_set_ops_byname is not
being checked in rte_mempool_ops_alloc.

This patch handles error returned from rte_mempool_set_ops_byname
when a mempool is not found.

Fixes: 449c49b93a6b ("mempool: support handler operations")
Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>