Fix buffer size in aead_test.c.

out2 wasn't sized to account for stateful AEAD open requiring a seal overhead's
worth of scratch space. Also, pass in sizeof(out2) rather than a computed
ciphertext length, so the max_out check would have actually caught this.

Change-Id: Ibe689424f6c8ad550b3a45266699892076e7ba5e
Reviewed-on: https://boringssl-review.googlesource.com/3060
Reviewed-by: Adam Langley <agl@google.com>