Re: Don't pass NULL pointer to strcmp().

Looking closer, I realized that my proposed change in RestoreGUCState
is unnecessary, because guc_free() is already permissive about being
passed a NULL.  That leaves us with one live bug in
get_explain_guc_options, two probably-unreachable hazards in
check_GUC_init and write_one_nondefault_variable, and two API changes
in GetConfigOption and GetConfigOptionResetString.  I'm dubious that
back-patching the API changes would be a good idea, so I applied
that to HEAD only.  The rest I backpatched as far as relevant.

Thanks for the report!

            regards, tom lane