Compile warning on Perl 5.14

[pali]

https://travis-ci.org/Dual-Life/Devel-PPPort/jobs/544798880

cc -c   -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2   -DVERSION=\"3.52\" -DXS_VERSION=\"3.52\" -fPIC "-I/home/travis/perl5/perlbrew/perls/5.14/lib/5.14.4/x86_64-linux/CORE"  -W -Wall -Wdeclaration-after-statement RealPPPort.c
In file included from RealPPPort.xs:142:0:
ppport.h: In function ‘DPPP_my_utf8_to_uvchr_buf’:
ppport.h:5012:35: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
             if (retlen && *retlen >= 0) {
                                   ^

CC @khwilliamson

[khwilliamson]

Fixed in PR #176