Sentinel: Add "dir /tmp" directive in example sentinel.conf.
fixed possible buffer overflow error
Fixed possible buffer overflow bug if RDB file is corrupted.

(Note: commit message modified by @antirez for clarity).
Fix 2.8 backport of fastscript branch.

No REDIS_ENCODING_EMBSTR in 2.8 internals.
Merge remote-tracking branch 'antirez/2.8' into 2.8