event: Relax restriction on g_reactors not being NULL

There is a case in vhost where this occurs. Until that is
sorted out, just make this a warning.

Change-Id: Id021791e8cbddf3023e0cb1b8c52a733b3578a7d
Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/466075
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Seth Howell <seth.howell@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

lib/event/reactor.c

@@ -93,7 +93,10 @@ spdk_reactor_get(uint32_t lcore)
 {
 	struct spdk_reactor *reactor;
 
-	assert(g_reactors != NULL);
+	if (g_reactors == NULL) {
+		SPDK_WARNLOG("Called spdk_reactor_get() while the g_reactors array was NULL!\n");
+		return NULL;
+	}
 
 	reactor = &g_reactors[lcore];