Make wpa_config_read_blob() easier for static analyzers

Make wpa_config_read_blob() easier for static analyzers

While encoded == NULL could happen in the case of an empty blob, that
will result in encoded_len == 0 and base64_decode() not derefencing the
src argument. That seems to be too difficult for some static analyzers,
so to avoid false warnings, explicitly reject the encoded == NULL case
without even trying to base64 decode it. (CID 164709)

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>