Switch an assert back to a check.

The assert was supposed to be *added* in fcf25833 but instead replaced
the check.

BUG=465557

Change-Id: I0d3db5038515021e5bdd1ccb9ff08d4f78552621
Reviewed-on: https://boringssl-review.googlesource.com/3850
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>