RFA: probable rs6000-aix-tdep.c bug found by clang

[Joel Brobecker]

> gdb/ChangeLog (by Tom Tromey and Joel Brobecker):
> 
>         * rs6000-aix-tdep.c (rs6000_aix_osabi_sniffer): Replace
>         inneffective if condition by gdb assertion.  Add function
>         description comment.

I confirm that testing revealed no issue, so checked in.
As usual on AIX, I've only been able to run the AdaCore testsuite,
which should be good enough for this particular change.

-- 
Joel