libdwfl: Fix some GCC10 -Wnull-dereference issues.

GCC10 on some arches will warn about possible NULL derefences.
In the libdwfl linux-kernel-modules.c cases it might be caught already
by earlier calls to get_release (). But it is hard to see that will
really always happen. So do an explicit NULL check just in case.

Signed-off-by: Mark Wielaard <mark@klomp.org>