bio_b64.c: prevent base64 filter BIO from decoding out-of-bound data
Fixes #5405, #1381 The base64 filter BIO reads its input in chunks of B64_BLOCK_SIZE bytes. When processing input in PEM format it can happen in rare cases that - the trailing PEM marker crosses the boundary of a chunk, and - the beginning of the following chunk contains valid base64 encoded data. This happened in issue #5405, where the PEM marker was split into "-----END CER" and "TIFICATE-----" at the end of the first chunk. The decoding of the first chunk terminated correctly at the '-' character, which is treated as an EOF marker, and b64_read() returned. However, when called the second time, b64_read() read the next chunk and interpreted the string "TIFICATE" as valid base64 encoded data, adding 6 extra bytes '4c 81 48 08 04 c4'. This patch restores the assignment of the error code to 'ctx->cont', which was deleted accidentally in commit 5562cfac and which prevents b64_read() from reading additional data on subsequent calls. This issue was observed and reported by Annie Yousar. Reviewed-by: Richard Levitte <levitte@openssl.org> (Merged from https://github.com/openssl/openssl/pull/5422)
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