Use HMAC_Init_ex, not HMAC_Init, in HMAC.

We've already initialized the context, HMAC_Init has questionable behavior
around NULL keys, and this avoids a size_t truncation.

Change-Id: Iab6bfc24fe22d46ca4c01be6129efe0630d553e6
Reviewed-on: https://boringssl-review.googlesource.com/3732
Reviewed-by: Adam Langley <agl@google.com>