bugfix(fmesher): fix out of bounds writes in writepoly.cpp

[convicted-pasta-thief]

Fixed a couple of out-of-bounds writes in writepoly.cpp. These cause crashes during the femmcli_TorqueBenchmark.lua and femmcli_antiperiodicBC_AGE_TorqueBenchmark.lua tests if the code is compiled using -D_GLIBCXX_ASSERTIONS.