ovsdb: Fix error leak for negative timeout and invalid until case

Although the check for negative timeout is present, the error string
is overwritten if an invalid "until" is found right after. This leaks
an error string and results in not reporting the negative timeout back
to the user even though it is encountered first.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>