Wrong dead return value in jsonb_utils.c

Returning -1 from a function with bool as return value is the same as
returning true. Now, the code is dead (since elog(ERROR, ...) does not
return) so it doesn't matter to the compiler, but changing to false is less
confusing for the programmer. Appologies if this is seen as unnecessary
churn.

The same code is present since 9.4, but perhaps it's not really worth
backporting since it is more of an aesthetic change?