Always reset sigalgslen when NULLing sigalgs.

See also upstream's 34e3edbf3a10953cb407288101fd56a629af22f9. This fixes
CVE-2015-0291. Also bubble up malloc failures in tls1_set_shared_sigalgs. Tidy
up style a bit and remove unnecessary check (it actually is unnecessary; see
https://boringssl-review.googlesource.com/4042).

Change-Id: Idfb31a90fb3e56ef6fe7701464748a5c1603f064
Reviewed-on: https://boringssl-review.googlesource.com/4047
Reviewed-by: Adam Langley <agl@google.com>