I realized looking at the sigalgs parser that I messed up the
space-splitting logic slightly. If the CPU features are "foo bar baz",
it would not parse "baz". This doesn't particular matter (the last one
is "crc32"), but better to parse it correctly.

Fix this and add a unit test. While I'm here, may as well add a fuzzer
too.

Change-Id: Ifc1603b8f70d975f391d10e51ede95deec31a83d
Reviewed-on: https://boringssl-review.googlesource.com/31464
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>