the conversion from int to size_t in charcnv did not take into account

the conversion from int to size_t in charcnv did not take into account
one place where we checked "if (src_len > 0)".

I actually would greatly prefer to switch back to int for src_len. The
type *can* be negative, which means an unsigned type is
inappropriate. There is absolutely no reason why "int" should not be
used for a parameter like this.

I didn't change back to int as we are close to a release and I wanted
a mininal change, but please don't go changing types like this in
future without very careful testing and a damn good reason.

this bug broke pull_ucs2(), I would not be surprised if it caused all
sorts of nastiness. Thanks to vl for noticing the symptoms!
(This used to be commit 8b8f0c527959c707f274b435fc59156e37232981)