claws-mail -- no bounds checking on the output buffer in conv_jistoeuc, conv_euctojis, conv_sjistoeuc

DrWhax reports:

So in codeconv.c there is a function for Japanese character set
conversion called conv_jistoeuc(). There is no bounds checking on
the output buffer, which is created on the stack with alloca()
Bug can be triggered by sending an email to [email protected]
or whatever.

    Since my C is completely rusty, you might be able to make a better
    judgment on the severity of this issue. Marking critical for now.