MemoryTracer: don't reconnect signal if prev and next module are not null

May end up with an infinite list of signals when more than one
module is monitored.

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>
s2e.h: killing state supports format strings

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>
savevm: reset buffer size or snapshot won't be restored properly

This bug clobbers the device state on state switches.
It would restore the device state of whatever execution state
was running previously.

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>