libbpf: Fix build failure from uninitialized variable warning

While compiling libbpf, some GCC versions (at least 8.4.0) have difficulty
determining control flow and a emit warning for potentially uninitialized
usage of 'map', which results in a build error if using "-Werror":

In file included from libbpf.c:56:
libbpf.c: In function '__bpf_object__open':
libbpf_internal.h:59:2: warning: 'map' may be used uninitialized in this function [-Wmaybe-uninitialized]
  libbpf_print(level, "libbpf: " fmt, ##__VA_ARGS__); \
  ^~~~~~~~~~~~
libbpf.c:5032:18: note: 'map' was declared here
  struct bpf_map *map, *targ_map;
                  ^~~

The warning/error is false based on code inspection, so silence it with a
NULL initialization.

Fixes: 646f02ff ("libbpf: Add BTF-defined map-in-map support")
Reference: 063e6881 ("libbpf: Fix false uninitialized variable warning")
Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200831000304.1696435-1-Tony.Ambardar@gmail.com

tools/lib/bpf/libbpf.c

@@ -5203,8 +5203,8 @@ static int bpf_object__collect_map_relos(struct bpf_object *obj,
 	int i, j, nrels, new_sz;
 	const struct btf_var_secinfo *vi = NULL;
 	const struct btf_type *sec, *var, *def;
+	struct bpf_map *map = NULL, *targ_map;
 	const struct btf_member *member;
-	struct bpf_map *map, *targ_map;
 	const char *name, *mname;
 	Elf_Data *symbols;
 	unsigned int moff;