r25238: Make the error returns from the string functions

r25238: Make the error returns from the string functions
always consistent. Return -1 on error, and ensure
we check for this. In cases where the dest is already
specified and we've been asked to terminate with a
null, ensure we always do so even on error.
Jeremy.
(This used to be commit abedd967869ade9a43c3a8e1b889c60d4aca81cf)