Fix crash bugs caused by incorrect type declaration

Hi. This pull request changes the type of variables used as the "length" argument to `SvPV` from `unsigned int` to `STRLEN`; this fixes a large class of crash bugs on typical 64-bit platforms.

I'm afraid I couldn't work out how to correctly navigate the build system used by this module, so I actually generated this patch by editing the code in the tarball on CPAN, and backporting it to your Git repo. That means I haven't directly tested the code in this pull request; it should work as far as I can tell, but please let me know if there are any problems.

Thanks.