Add bounds check in PKCS7 streaming indefinite-length end-of-content parsing

[anhu]

Fixes ZD 21399 Finding #1

[anhu]

Jenkins retest this please.

[dgarske]

Clang-tidy: unchecked XFSEEK return values in test_pkcs7.c lines 4878 and 4880 — the PR adds calls like XFSEEK(f, 0, XSEEK_END); and XFSEEK(f, 0, XSEEK_SET); without using the return value, triggering bugprone-unused-return-value. This failed 3 of 4 clang-tidy configurations (PRB-multi-test-script #9983).

Fix: capture and check the return value, e.g.:

if (XFSEEK(f, 0, XSEEK_END) != 0) { /* handle error */ }