sandbox: bugfix infinite loop in new_ttrpc_client() #908

Job	Run time
tests (vmm/sandbox, --all-features)	3m 14s
checks (vmm/sandbox, --all-features)	4m 21s
tests (vmm/task, --all-features)	2m 33s
checks (vmm/task, --all-features)	3m 54s
tests (shim, --all-features)	2m 33s
checks (shim, --all-features)	3m 57s
tests (quark, --all-features)	2m 26s
checks (quark, --all-features)	3m 44s
checks (runc, --all-features)	3m 53s
checks (wasm, --features=wasmedge, 0.11.2)	3m 48s
tests (runc, --all-features)	2m 28s
checks (wasm, --features=wasmtime)	4m 11s
tests (wasm, --features=wasmedge, 0.11.2)	2m 34s
tests (wasm, --features=wasmtime)	3m 25s
	47m 1s

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