Fix parameter type of p256-64.c's |select_point|.

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>

crypto/ec/p256-64.c

@@ -1404,7 +1404,8 @@ static const smallfelem g_pre_comp[2][16][3] = {
 /* select_point selects the |idx|th point from a precomputation table and
  * copies it to out. */
 static void select_point(const u64 idx, unsigned int size,
-                         const smallfelem pre_comp[16][3], smallfelem out[3]) {
+                         const smallfelem pre_comp[/*size*/][3],
+                         smallfelem out[3]) {
   unsigned i, j;
   u64 *outlimbs = &out[0][0];
   memset(outlimbs, 0, 3 * sizeof(smallfelem));