claws-mail: buffer overflow

A remotely triggerable buffer overflow has been found in the code of
claws-mail handling character conversion, in functions conv_jistoeuc(),
conv_euctojis() and conv_sjistoeuc(), in codeconv.c.
There was no bounds checking on buffers passed to these functions, some
stack-based but other potentially heap-based.
This issue has been located in the wild and might currently be exploited.