Fix off by one bug in freeMemoryIfNeeded() eviction pool.

Bug found by the continuous integration test running the Redis
with valgrind:

==6245== Invalid read of size 8
==6245==    at 0x4C2DEEF: memcpy@GLIBC_2.2.5 (mc_replace_strmem.c:876)
==6245==    by 0x41F9E6: freeMemoryIfNeeded (redis.c:3010)
==6245==    by 0x41D2CC: processCommand (redis.c:2069)

memmove() size argument was accounting for an extra element, going
outside the bounds of the array.