The expectation when calling HMAC with key=NULL and keylen=0 is to compute
HMAC on the provided data with a key of length 0 instead of using the
"previous" key, which in the case of HMAC() is whatever bytes happen to be
left on the stack when the HMAC_CTX struct is allocated.

Change-Id: I52a95e262ee4e15f1af3136cb9c07f42f40ce122
Reviewed-on: https://boringssl-review.googlesource.com/2660
Reviewed-by: Adam Langley <agl@google.com>