Make fe_is_zero side-effect free

Author: sipa

src/field_10x26_impl.h

@@ -275,10 +275,9 @@ SECP256K1_INLINE static void secp256k1_fe_set_int(secp256k1_fe *r, int a) {
 
 SECP256K1_INLINE static int secp256k1_fe_is_zero(const secp256k1_fe *a) {
     const uint32_t *t = a->n;
-#ifdef VERIFY
-    VERIFY_CHECK(a->normalized);
-    secp256k1_fe_verify(a);
-#endif
+    /* No secp256k1_fe_verify is needed here, because if all limbs are 0,
+     * the representation is always valid. This also allows using this function
+     * inside VERIFY_CHECK conditions itself without side effects. */
     return (t[0] | t[1] | t[2] | t[3] | t[4] | t[5] | t[6] | t[7] | t[8] | t[9]) == 0;
 }
 

src/field_5x52_impl.h

@@ -238,10 +238,9 @@ SECP256K1_INLINE static void secp256k1_fe_set_int(secp256k1_fe *r, int a) {
 
 SECP256K1_INLINE static int secp256k1_fe_is_zero(const secp256k1_fe *a) {
     const uint64_t *t = a->n;
-#ifdef VERIFY
-    VERIFY_CHECK(a->normalized);
-    secp256k1_fe_verify(a);
-#endif
+    /* No secp256k1_fe_verify is needed here, because if all limbs are 0,
+     * the representation is always valid. This also allows using this function
+     * inside VERIFY_CHECK conditions itself without side effects. */
     return (t[0] | t[1] | t[2] | t[3] | t[4]) == 0;
 }