Ensure that we never write an out of bounds value (_bufend) to  _begin or _end, even temporarily.
Ensure that we never write an out of bounds value (_bufend) to  _begin or _end, even temporarily.

Testing:
  - Boot tested, ran basic serial I/O code

Notes:
  - Before this change, there are instruction like "s32i.n <reg>, <this>, <_begin>" in the
    disassembled output, followed by an overwrite if <reg> turns out to be _bufend.
    After this change, there is only one store instruction to <_begin> per function.

toastedcode/esp8266-Arduino,toastedcode/esp8266-Arduino,gguuss/Arduino,quertenmont/Arduino,gguuss/Arduino,hallard/Arduino,toastedcode/esp8266-Arduino,edog1973/Arduino,Lan-Hekary/Arduino,Adam5Wu/Arduino,lrmoreno007/Arduino,NullMedia/Arduino,Juppit/Arduino,me-no-dev/Arduino,gguuss/Arduino,martinayotte/ESP8266-Arduino,quertenmont/Arduino,KaloNK/Arduino,Juppit/Arduino,Juppit/Arduino,Juppit/Arduino,NullMedia/Arduino,NullMedia/Arduino,wemos/Arduino,jes/Arduino,lrmoreno007/Arduino,jes/Arduino,gguuss/Arduino,esp8266/Arduino,me-no-dev/Arduino,NextDevBoard/Arduino,edog1973/Arduino,NullMedia/Arduino,sticilface/Arduino,hallard/Arduino,NextDevBoard/Arduino,Links2004/Arduino,esp8266/Arduino,Adam5Wu/Arduino,sticilface/Arduino,wemos/Arduino,edog1973/Arduino,Lan-Hekary/Arduino,gguuss/Arduino,NextDevBoard/Arduino,Adam5Wu/Arduino,me-no-dev/Arduino,sticilface/Arduino,KaloNK/Arduino,sticilface/Arduino,edog1973/Arduino,wemos/Arduino,Lan-Hekary/Arduino,sticilface/Arduino,NullMedia/Arduino,Links2004/Arduino,martinayotte/ESP8266-Arduino,jes/Arduino,hallard/Arduino,martinayotte/ESP8266-Arduino,Lan-Hekary/Arduino,Links2004/Arduino,lrmoreno007/Arduino,me-no-dev/Arduino,esp8266/Arduino,me-no-dev/Arduino,esp8266/Arduino,Links2004/Arduino,KaloNK/Arduino,toastedcode/esp8266-Arduino,quertenmont/Arduino,hallard/Arduino,lrmoreno007/Arduino,hallard/Arduino,esp8266/Arduino,Adam5Wu/Arduino,toastedcode/esp8266-Arduino,KaloNK/Arduino,quertenmont/Arduino,edog1973/Arduino,jes/Arduino,jes/Arduino,KaloNK/Arduino,Links2004/Arduino,wemos/Arduino,Lan-Hekary/Arduino,NextDevBoard/Arduino,Adam5Wu/Arduino,wemos/Arduino,martinayotte/ESP8266-Arduino,martinayotte/ESP8266-Arduino,quertenmont/Arduino,lrmoreno007/Arduino,NextDevBoard/Arduino,Juppit/Arduino