libdwfl: fix potential NULL pointer dereference when reading link map

libdwfl: fix potential NULL pointer dereference when reading link map

When read_addrs() was moved into file scope, there was a mistake in
converting "buffer" from a closure variable to a parameter: we are
checking whether the pointer argument is NULL, not whether the buffer
itself is NULL.  This causes a NULL pointer dereference when we try
to use the NULL buffer later.

Fixes: 3bf41d458fb6 ("link_map: Pull read_addrs() into file scope")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>