bpftool: Fix maybe-uninitialized warnings

[ Upstream commit 4bbb3583687051ef99966ddaeb1730441b777d40 ]

Somehow when bpftool is compiled in -Og mode, compiler produces new warnings
about possibly uninitialized variables. Fix all the reported problems.

Fixes: 2119f2189d ("bpftool: add C output format option to btf dump subcommand")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210313210920.1959628-3-andrii@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>

tools/bpf/bpftool/map.c

@@ -99,7 +99,7 @@ static int do_dump_btf(const struct btf_dumper *d,
 		       void *value)
 {
 	__u32 value_id;
-	int ret;
+	int ret = 0;
 
 	/* start of key-value pair */
 	jsonw_start_object(d->jw);