test/nvmf: call killprocess only when nvmfpid exists

In nvmftestfini we try to kill the nvmf application, but this doesn't
get set by nvmftestinit. I think that nvmftestfini and nvmftestinit
should be symmetric options, or at the very least, one should be able to
call fini right after calling init without fini failing.

Change-Id: I14f274f940ec20c2d5f8d831fed3f6d6c4c6d2ca
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/476687


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Community-CI: Broadcom SPDK FC-NVMe CI <spdk-ci.pdl@broadcom.com>