pgsql: Code review for pgstat_get_crashed_backend_activity patch.

Code review for pgstat_get_crashed_backend_activity patch.

Avoid possibly dumping core when pgstat_track_activity_query_size has a
less-than-default value; avoid uselessly searching for the query string
of a successfully-exited backend; don't bother putting out an ERRDETAIL if
we don't have a query to show; some other minor stylistic improvements.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/f9c92a5a3ead738c7de0dffa203a92b4d2fec413

Modified Files
--------------
src/backend/postmaster/pgstat.c     |   54 ++++++++++++++++++++--------------
src/backend/postmaster/postmaster.c |   25 +++++++++------
src/backend/utils/adt/ascii.c       |   29 ++++++++++--------
src/include/pgstat.h                |    2 +-
src/include/utils/ascii.h           |    3 +-
5 files changed, 66 insertions(+), 47 deletions(-)