Comment and check a parameter for inf in secp256k1_ecmult_const.

Author: roconnor-blockstream

src/ecmult_const.h

@@ -14,6 +14,7 @@
  * Multiply: R = q*A (in constant-time)
  * Here `bits` should be set to the maximum bitlength of the _absolute value_ of `q`, plus
  * one because we internally sometimes add 2 to the number during the WNAF conversion.
+ * A must not be infinity.
  */
 static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *q, int bits);
 

src/ecmult_const_impl.h

@@ -168,6 +168,7 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
      * that the Z coordinate was 1, use affine addition formulae, and correct
      * the Z coordinate of the result once at the end.
      */
+    VERIFY_CHECK(!a->infinity);
     secp256k1_gej_set_ge(r, a);
     secp256k1_ecmult_odd_multiples_table_globalz_windowa(pre_a, &Z, r);
     for (i = 0; i < ECMULT_TABLE_SIZE(WINDOW_A); i++) {