Fixed build warning of signed/unsigned conditional expression

When building mongoose with this command line

gcc -c -m64 -Wall -W mongoose.c -o mongoose

the following build warning appears:

mongoose.c: In function ‘forward_put_data’:
mongoose.c:3559:54: warning: signed and unsigned type in conditional expression [-Wsign-compare]
   size_t k = conn->cl < (int64_t) io->len ? conn->cl : io->len;   // To write