hello_world: fail if no NVMe devices found

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I90feac0377b4870c1552df3eb470e834ceae4f65

Reviewed-on: https://review.gerrithub.io/362265
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

examples/nvme/hello_world/hello_world.c

@@ -334,6 +334,12 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
+	if (g_controllers == NULL) {
+		fprintf(stderr, "no NVMe controllers found\n");
+		cleanup();
+		return 1;
+	}
+
 	printf("Initialization complete.\n");
 	hello_world();
 	cleanup();