Avoid array index overrun when clearing bss section in ROM
Avoid array index overrun when clearing bss section in ROM

p_memsz for .data+.bss takes into account the space required for .bss
values; attempting to zero the ROM area which won't be copied in can overrun
the address space.  The fill probably shouldn't even be run.

Signed-off-by: Joakim Eriksson <c384721d08d3df01e04c8312462a259306d13759@sics.se>

contiki-os/mspsim,joakimeriksson/mspsim,nfi/mspsim,fvdnabee/mspsim,contiki-os/mspsim,joakimeriksson/mspsim,ransford/mspsim,nfi/mspsim,mspsim/mspsim,mspsim/mspsim,ransford/mspsim,cmorty/mspsim,fvdnabee/mspsim,cmorty/mspsim