src/tconv.c: fix wrong placement for output buffer management, do increase of output buffer by step of TCONV_HELPER_BUFSIZ instead of *= 2
src/tconv.c: fix wrong placement for output buffer management, do increase of output buffer by step of TCONV_HELPER_BUFSIZ instead of *= 2

jddurand/c-tconv,jddurand/c-tconv,jddurand/c-tconv,jddurand/c-tconv