[lib] Fix the struct message LOOKUP function to be more robust

[lib] Fix the struct message LOOKUP function to be more robust

2008-02-28 Paul Jakma <paul.jakma@sun.com>

	* log.c: (mes_lookup) Sowmini Varadhan diagnosed a problem where
	  this function can cause a NULL dereference, on lookups for unknown
	  indices, or messages with NULL strings. Can occur, e.g., debug
	  logging code when processing received messages. Fixed to accept a
	  pointer to a default string to be used if there is no match.
	* log.h: LOOKUP adjusted to match