pgsql: Fix integer overflow in debug message of walreceiver

Fix integer overflow in debug message of walreceiver

The message tries to tell the replication apply delay which fails if
the first WAL record is not applied yet. Fix is, instead of telling
overflowed minus numeric, showing "N/A" which indicates that the delay
data is not yet available. Problem reported by me and patch by
Fabrízio de Royes Mello.

Back patched to 9.4, 9.3 and 9.2 stable branches (9.1 and 9.0 do not
have the debug message).

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/4909cb59df40a011b041ce97a68e4d73a54da6ee

Modified Files
--------------
src/backend/replication/walreceiver.c | 21 ++++++++++++++++-----
src/backend/replication/walreceiverfuncs.c | 12 ++++++++++--
2 files changed, 26 insertions(+), 7 deletions(-)