pgsql: Fix elog.c to avoid infinite recursion (leading to backend crash)

Log Message:
-----------
Fix elog.c to avoid infinite recursion (leading to backend crash) when
log_min_error_statement is active and there is some problem in logging the
current query string; for example, that it's too long to include in the log
message without running out of memory. This problem has existed since the
log_min_error_statement feature was introduced. No doubt the reason it
wasn't detected long ago is that 8.2 is the first release that defaults
log_min_error_statement to less than PANIC level.
Per report from Bill Moran.

Modified Files:
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pgsql/src/backend/utils/error:
elog.c (r1.189 -> r1.190)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.189&r2=1.190)