selftests/bpf: Fix few more compiler warnings

When compiling with -O2, GCC detects few problems with selftests/bpf, so
fix all of them. Two are real issues (uninitialized err and nums
out-of-bounds access), but two other uninitialized variables warnings
are due to GCC not being able to prove that variables are indeed
initialized under conditions under which they are used.

Fix all 4 cases, though.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20220705224818.4026623-3-andrii@kernel.org

tools/testing/selftests/bpf/prog_tests/xdp_synproxy.c

@@ -63,7 +63,7 @@ static bool expect_str(char *buf, size_t size, const char *str, const char *name
 static void test_synproxy(bool xdp)
 {
 	int server_fd = -1, client_fd = -1, accept_fd = -1;
-	char *prog_id, *prog_id_end;
+	char *prog_id = NULL, *prog_id_end;
 	struct nstoken *ns = NULL;
 	FILE *ctrl_file = NULL;
 	char buf[CMD_OUT_BUF_SIZE];