tpmtest: add a missed size initialization
tpmtest: add a missed size initialization

This might cause random failures.

01org/TPM2.0-TSS,01org/tpm2-tss,01org/tpm2-tss,tpm2-software/tpm2-tss,tpm2-software/tpm2-tss,01org/TPM2.0-TSS,tpm2-software/tpm2-tss,01org/tpm2-tss,01org/TPM2.0-TSS,01org/TPM2.0-TSS