chunked decoder: track overflows correctly

The code didn't properly check the return codes to detect overflows so
it could trigger incorrectly. Like on mingw32.

Regression introduced in 345891edba (curl 7.35.0)

Bug: http://curl.haxx.se/mail/lib-2014-02/0097.html
Reported-by: LM
Fix compilation with make mingw32

The source files from lib/vtls where generated in lib instead of lib/vtls.

Verified-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>