Bugfix for potential flash buffer overrun in avr_loadcode (sim_avr.c)

In function avr_loadcode (sim_avr.c) a memcpy is performed to load code
        into flash at address of size block...  However, only the size is
	currently checked against avr->flashend potentially leading to a
	block overrun.

	modified:   simavr/sim/sim_avr.c