Fix a couple of bugs for asym crypto introduced in r359374.

[jhb]

Summary

• Check for null pointers in the crypto_drivers[] array when checking for empty slots in crypto_select_kdriver().

• Handle the case where crypto_kdone() is invoked on a request where krq_cap is NULL due to not finding a matching driver.

Test Plan

• run src/tools/tools/cryptokeytest, it panicked until these were fixed

[markj]

sys/opencrypto/crypto.c

1543 ↗

(On Diff #78291)

This can be one line.

[jhb]

sys/opencrypto/crypto.c

1543 ↗

(On Diff #78291)

Yes, it is still split out as two lines in the similar function for symmetric drivers which is why I left it this way. I might do a followup to fix both functions though.