elf: Avoid some free (NULL) calls in _dl_update_slotinfo

This has been confirmed to work around some interposed mallocs.  Here
is a discussion of the impact test ust/libc-wrapper/test_libc-wrapper
in lttng-tools:

  New TLS usage in libgcc_s.so.1, compatibility impact
  <https://inbox.sourceware.org/libc-alpha/8734v1ieke.fsf@oldenburg.str.redhat.com/

>

Reportedly, this patch also papers over a similar issue when tcmalloc
2.9.1 is not compiled with -ftls-model=initial-exec.  Of course the
goal really should be to compile mallocs with the initial-exec TLS
model, but this commit appears to be a useful interim workaround.

Fixes commit d2123d68 ("elf: Fix slow
tls access after dlopen [BZ #19924]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit afe42e93)