libdw: Don't pass NULL to dwarf_peel_type

libdw: Don't pass NULL to dwarf_peel_type

commit c3a6a9dfc "libdw: Use signedness of subrange type to
determine array bounds" introduced a type check on a DIE which
exposed a latent bug in the get_type function. Even if the type
of a DIE couldn't be determined it would call dwarf_peel_type
on it. The gcc undefined sanitizer would flag this as being
undefined behaviour because the second argument of the function
is marked as non-NULL. Fix this by checking we actually have
a non-NULL type DIE.

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