Use STRLEN type where required

On platforms with 64-bit pointers and 32-bit ints, declaring the variable
in which SvPV stores the SV's length as an `unsigned int` rather than a
`STRLEN` causes a wild memory write. If Perl is compiled using GCC's
-fstack-protector-strong option, this further causes an immediate abort
when executing at least some affected code.
Merge pull request #22 from arc/master

Fix crash bugs caused by incorrect type declaration