Merge bitcoin-core/secp256k1#1764: group: Avoid using infinity field directly in other modules

2f73e5281d group: Avoid using infinity field directly in other modules (Tim Ruffing)

Pull request description:

  Minor refactoring to make the abstraction cleaner

ACKs for top commit:
  hebasto:
    ACK 2f73e5281d, I have reviewed the code and it looks OK.
  theStack:
    ACK 2f73e5281d

Tree-SHA512: eae5ad1ce81f491adb48ab1cbf04211f8d43e41255abcacc958fa3dcb1de5021707d56ed1b009a6f3f6c45cd8f20c1f2677891690a3c0a467fc7e064af2512a8

src/ecmult_impl.h

@@ -75,7 +75,7 @@ static void secp256k1_ecmult_odd_multiples_table(int n, secp256k1_ge *pre_a, sec
     secp256k1_ge d_ge;
     int i;
 
-    VERIFY_CHECK(!a->infinity);
+    VERIFY_CHECK(!secp256k1_gej_is_infinity(a));
 
     secp256k1_gej_double_var(&d, a, NULL);
 
@@ -356,7 +356,7 @@ static void secp256k1_ecmult_strauss_wnaf(const struct secp256k1_strauss_state *
         }
     }
 
-    if (!r->infinity) {
+    if (!secp256k1_gej_is_infinity(r)) {
         secp256k1_fe_mul(&r->z, &r->z, &Z);
     }
 }