[Bug 253081] fdlopen is broken in STABLE-12

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=253081

            Bug ID: 253081
           Summary: fdlopen is broken in STABLE-12
           Product: Base System
           Version: 12.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: misc
          Assignee: bugs at FreeBSD.org
          Reporter: theraven at FreeBSD.org

I am not sure exactly when this happened because I'd forgotten that I was
carrying a local patch to rtld to fix it for a while and reverted to a pristine
upstream after the new git migration to be reapply my patches.

rtld fails in _rtld_bind with a library opened with fdlopen on this line:

https://github.com/freebsd/freebsd-src/blob/f56d7f838f5b3aa0f55b10406eaa7eb760a3ba18/libexec/rtld-elf/rtld.c#L879

This line and the basename implementation it called appear not to have been
modified in the last 23 years, so the root cause is, unfortunately, somewhere
else (it's not actually clear to me that this code ever worked, unless perhaps
`fdlopen` was never tested without `RTLD_NOW`?).  

Removing this line causes my code to work again but with it the call to
`basename(obj->path`) crashes in the `strrchr` call because `obj->path` is
null.  I believe this is the expected behaviour with fdlopen, because the file
descriptor (in my case, inherited from a parent).  I have had a local patch to
the `basename` implementation to handle being passed a null argument and return
null, but this caller seems to be the only one that triggers it and so it's
probably better to not call `basename` with a NULL argument on that line.

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