hydra: fix segfaults in argument parsing

[hzhou]

Pull Request Description

Add checks for missing argument. As a TODO, we should enhance the error
reporting to make it more user friendly. But we should prevent segfault
for now at least.

Fixes #5594

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[hzhou]

test:mpich/ch3/tcp

[hzhou]

The fixed behavior:

$ mpirun
[mpiexec@Tiger] HYDU_create_proxy_list (utils/alloc/alloc.c:436): Missing executable.
[mpiexec@Tiger] main (ui/mpich/mpiexec.c:263): unable to create proxy list

$ mpirun -n
[mpiexec@Tiger] np_fn (ui/mpich/utils.c:788): missing command line argument, add -h for help
[mpiexec@Tiger] match_arg (utils/args/args.c:156): match handler returned error
[mpiexec@Tiger] HYDU_parse_array (utils/args/args.c:178): argument matching returned error
[mpiexec@Tiger] parse_args (ui/mpich/utils.c:1639): error parsing input array
[mpiexec@Tiger] HYD_uii_mpx_get_parameters (ui/mpich/utils.c:1691): unable to parse user arguments
[mpiexec@Tiger] main (ui/mpich/mpiexec.c:127): error parsing parameters

[raffenet]

Is the idea to make this a macro so it can be added to other argument parsing functions?

[hzhou]

Yes.