F-6765: scrub previous value before overwriting PSA storage

ARM_TEE_PS_SET on an existing object XMEMCPY'd the new value over the
old one without clearing the buffer first, so a SET that stores less
data (or zero data) left the tail of the previous object readable
through ARM_TEE_PS_GET.

Zero the full data area with wc_ForceZero after every validation check
passes and before the copy, matching the DELETE path.

Reported by Fenrir.

src/arm_tee_psa_ipc.c

@@ -925,6 +925,10 @@ static int32_t arm_tee_psa_ps_dispatch(int32_t type, const psa_invec *in_vec,
         if (data_len > 0 && data == NULL) {
             return PSA_ERROR_INVALID_ARGUMENT;
         }
+        /* Scrub the previous value before overwriting: a SET that stores
+         * less data (or zero) must not leave the tail of the old object
+         * readable via GET. Runs only after every validation check. */
+        wc_ForceZero(entry->data, sizeof(entry->data));
         if (data_len > 0) {
             XMEMCPY(entry->data, data, data_len);
         }