libbpf: Fix false uninitialized variable warning

Some versions of GCC falsely detect that vi might not be initialized. That's
not true, but let's silence it with NULL initialization.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200430021436.1522502-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -5003,8 +5003,8 @@ static int bpf_object__collect_map_relos(struct bpf_object *obj,
 					 GElf_Shdr *shdr, Elf_Data *data)
 {
 	int i, j, nrels, new_sz, ptr_sz = sizeof(void *);
+	const struct btf_var_secinfo *vi = NULL;
 	const struct btf_type *sec, *var, *def;
-	const struct btf_var_secinfo *vi;
 	const struct btf_member *member;
 	struct bpf_map *map, *targ_map;
 	const char *name, *mname;