env_dpdk: add an assert check for last core

Change-Id: If2a6292101e5c267908cf2b52b8c9407fdbd9fc0
Signed-off-by: Ziye Yang <optimistyzy@gmail.com>
Reviewed-on: https://review.gerrithub.io/400908
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>

lib/env_dpdk/threads.c

@@ -64,6 +64,8 @@ spdk_env_get_last_core(void)
 		last_core = i;
 	}
 
+	assert(last_core != UINT32_MAX);
+
 	return last_core;
 }