pgsql: Fix bogus Name assignment in CreateStatistics

Fix bogus Name assignment in CreateStatistics

Apparently, it doesn't work to use a plain cstring as a Name datum: you
may end up having random bytes because of failing to zero the bytes
after the terminating \0, as indicated by valgrind. I introduced this
bug in 5564c1181548, so backpatch this fix to REL_10_STABLE, like that
commit.

While at it, fix a slightly misleading comment, pointed out by David
Rowley.

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e20dd6a13d870f5c98a163031b38ba23753e628c

Modified Files
--------------
src/backend/commands/statscmds.c | 7 +++++--
src/backend/parser/parse_utilcmd.c | 3 ++-
2 files changed, 7 insertions(+), 3 deletions(-)