Make secp256k1_ecmult_const handle infinity

Infinity isn't currently needed here, but correctly handling it is a
little more safe against future changes.

Update docs for it to make it clear that it is not constant time in A
(the input point). It never was constant time in Q (and would be a little
complicated to make constant time in A).

If it was later made constant time in A, infinity support would be easy
to preserve, e.g. by running it on a dummy value and cmoving infinity into
the output.

src/ecmult_const_impl.h

@@ -144,6 +144,11 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
 
     int i;
 
+    if (secp256k1_ge_is_infinity(a)) {
+        secp256k1_gej_set_infinity(r);
+        return;
+    }
+
     /* build wnaf representation for q. */
     /* split q into q_1 and q_lam (where q = q_1 + q_lam*lambda, and q_1 and q_lam are ~128 bit) */
     secp256k1_scalar_split_lambda(&q_1, &q_lam, scalar);