GCC4.6: Squash warning in cmd_mem.c

cmd_mem.c: In function ‘do_mem_loop’:
cmd_mem.c:474:25: warning: variable ‘junk’ set but not used
[-Wunused-but-set-variable]

The assigned variable can be removed because the pointers are volatile so
accesses to their addresses are always generated.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>