hostapd: ucode: fix per-station PSK list parsing in sta_auth

hostapd: ucode: fix per-station PSK list parsing in sta_auth

Two defects handling the psk array returned by an auth handler: strlen()
ran before the string was validated (strlen(NULL) on a non-string entry),
and the hex-PMK branch tested the array length instead of the entry length.
A 64-character passphrase therefore reached the passphrase branch and its
memcpy of str_len + 1 overflowed passphrase[MAX_PASSPHRASE_LEN + 1] by one
byte. Validate the type first and branch on the string length.

Signed-off-by: Felix Fietkau <nbd@nbd.name>