ovn: Fix extra token detection.

ovn: Fix extra token detection.

This code attempts to first check whether another error was detected for
the string it is parsing, then if it's not at the end of the tokens,
report an error. However, 'errorp' is always a valid pointer to a
'char *', so the first check in this statement always evaluates false.

Furthermore, this undefined behaviour may be optimised out by modern
compilers due to the prior dereference in expr_parse(). Fix this to
check the actual value of *errorp. Also add a test to check this case.

Found by MIT STACK undefined behaviour checker.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>