bpf: Fix passing zero to PTR_ERR() in bpf_btf_printf_prepare

There is a bug when passing zero to PTR_ERR() and return.
Fix the smatch error.

Fixes: c4d0bfb4 ("bpf: Add bpf_snprintf_btf helper")
Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/1604735144-686-1-git-send-email-wangqing@vivo.com