cryptsetup: avoid a segfault when a keyfile is passed along with a TPM device

cryptsetup: avoid a segfault when a keyfile is passed along with a TPM device

When a keyfile is passed with tpm2-device=, e.g.,

 systemd-cryptsetup attach test_data /vol /my-pass tpm2-device=auto

the logic in attach_luks_or_plain_or_bitlk_by_tpm2() tries to process it as a
TPM blob first. This did not work properly because it passes n_blobs=1 to
acquire_tpm2_key(), and the key_file is only read when n_blobs == 0. As a
result, the code ends up calling tpm2_unseal(..., blobs=NULL, n_blobs=1, ...).

Before commit 5c6aad9 ("cryptsetup-tokens: Print tpm2-primary-alg: only when it
is known"), the segfault was not observed because tpm2_unseal() was bailing out
early when primary_alg == 0. However, after that change, it attempts to process
the blob (which is NULL) and crashes.

Fix this logic by passing n_blobs=0 to acquire_tpm2_key() so that it actually
reads the key_file. Additionally, assert 'blobs' in tpm2_unseal() as a
safeguard.

Fixes #41867