[patch] Fix possible memory corruption in bogofilter 1.2.1

[Julius Plenz]

Hi,

I found a bug in the base64_decode function which may cause memory
corruption when the function is executed on a malformed base64 encoded
string.

Please find attached a minimal example which makes "bogofilter -e -p"
crash and a patch (with inline problem description) that fixes the bug.

Cheers,
Julius
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