Fix a couple more malloc test crashes.

The ex_data index may fail to be allocated. Also don't leave a dangling pointer
in handshake_dgst if EVP_DigestInit_ex fails and check a few more init function
failures.

Change-Id: I2e99a89b2171c9d73ccc925a2f35651af34ac5fb
Reviewed-on: https://boringssl-review.googlesource.com/2342
Reviewed-by: Adam Langley <agl@google.com>