Since the "pass" option supports both strings and variables, a generic
nxt_var_t structure can be used in the configuration phase, and the "name"
field in actions is redundant.
No functional changes.
In the case that routes or upstreams is empty and the pass option is a variable.
If the resolved pass is routes or upstreams, a segment error occurred.
Messed up return values in nxt_upstream_find() caused error in applying any
configuration with a valid "pass" value in router configuration pointing to
upstream. That wasn't the case in "listeners" objects, where the return value
wasn't checked.
Also, it caused segfault in cases where the "pass" option was configured with
variables and resulting value was pointing to a non-existent upstream.
Added missing return checks as well to catch possible memory allocation errors.
The bug was introduced in d32bc428f46b.
This closes#472 issue on GitHub.
