sandbox: bugfix infinite loop in new_ttrpc_client() #907

Job	Run time
tests (vmm/sandbox, --all-features)	3m 28s
tests (vmm/task, --all-features)	2m 35s
checks (vmm/sandbox, --all-features)	4m 33s
tests (shim, --all-features)	2m 38s
checks (vmm/task, --all-features)	3m 50s
tests (quark, --all-features)	2m 22s
checks (shim, --all-features)	3m 55s
tests (runc, --all-features)	2m 39s
checks (quark, --all-features)	4m 1s
tests (wasm, --features=wasmedge, 0.11.2)	2m 42s
tests (wasm, --features=wasmtime)	3m 6s
checks (runc, --all-features)	3m 57s
checks (wasm, --features=wasmedge, 0.11.2)	3m 54s
checks (wasm, --features=wasmtime)	4m 22s
	48m 2s

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