syntax-check: fix violations and re-enable sc_avoid_if_before_free.

syntax-check: fix violations and re-enable sc_avoid_if_before_free.

* cfg.mk (local-checks-to-fix): Remove sc_avoid_if_before_free
from list of disabled checks.
According to gnulib/doc/free.texi: "On old platforms such as
SunOS4, @code{free (NULL)} fails.  However, since all such
systems are so old as to no longer be considered ``reasonable
portability targets,'' this module is no longer useful."
* libltdl/libltdl/lt__alloc.h (FREE, MEMREASSIGN): Don't protect
against freeing NULL bug on SunOS4.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>