Make it match how it is done in p224-64.c. Note in particular that
|size| may be 17, so presumably |pre_comp[16]| is accessed, which one
would not expect when it was declared |precomp[16][3]|.
Change-Id: I54c1555f9e20ccaacbd4cd75a7154b483b4197b7
Reviewed-on: https://boringssl-review.googlesource.com/7467
Reviewed-by: David Benjamin <davidben@google.com>