Fix segfault in per_node_error_log() on armhf architecture.

Fix segfault in per_node_error_log() on armhf architecture.

pool_extract_error_message() incorrectly returns 255 (in decimal) on
the architecture when previous message was not an error or a notice
message. In this case per_node_error_log() happily calls ereport since
the return value from pool_extract_error_message() is greater than
0. Unfortunately the message string returned by
pool_extract_error_message() points to garbage memory in this case, a
segfault occurs.

The fix gives per_node_error_log() a guard against the bug of
pool_extract_error_message(). Moreover, the change is more consistent
with other places where pool_extract_error_message() is called.

Fix for pool_extract_error_message() will come later on.

See:
https://github.com/pgpool/pgpool2/issues/14
for more detailed discussion.

Problem reported and patch by Christian Ehrhardt.