libbpf: Fix handling of optional field_name in btf_dump__emit_type_decl

Internal functions, used by btf_dump__emit_type_decl(), assume field_name is
never going to be NULL. Ensure it's always the case.

Fixes: 9f81654e ("libbpf: Expose BTF-to-C type declaration emitting API")
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200303180800.3303471-1-andriin@fb.com

tools/lib/bpf/btf_dump.c

@@ -1030,7 +1030,7 @@ int btf_dump__emit_type_decl(struct btf_dump *d, __u32 id,
 	if (!OPTS_VALID(opts, btf_dump_emit_type_decl_opts))
 		return -EINVAL;
 
-	fname = OPTS_GET(opts, field_name, NULL);
+	fname = OPTS_GET(opts, field_name, "");
 	lvl = OPTS_GET(opts, indent_level, 0);
 	btf_dump_emit_type_decl(d, id, fname, lvl);
 	return 0;