env_dpdk/memory: silence scan-build errors

Scan-build complains that spdk_vtophys_notify() can segfault
on vaddr == 0. We know that and it's not a programming error.
In fact, SPDK with UIO pci driver can segfault upon registering
any non-mapped memory address. The user is just not supposed
to do that.

Assert vaddr != 0 to silence scan-build.

Change-Id: I9a14696361eca0aeea5ede9f9f2956fcbf59bdb5
Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/462869


Reviewed-by: yidong0635 <dongx.yi@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>