Avoid an out-of-bounds read in the redis-sentinel (#7443)

The Redis sentinel would crash with a segfault after a few minutes
because it tried to read from a page without read permissions. Check up
front whether the sds is long enough to contain redis:slave or
redis:master before memcmp() as is done everywhere else in
sentinelRefreshInstanceInfo().

Bug report and commit message from Theo Buehler. Fix from Nam Nguyen.

Co-authored-by: Nam Nguyen <namn@berkeley.edu>