Improve VERIFY_CHECK of overflow in secp256k1_scalar_cadd_bit.

This added check ensures that any curve order overflow doesn't go undetected due a uint32_t overflow.

Author: roconnor-blockstream

src/scalar_low_impl.h

@@ -40,6 +40,9 @@ static void secp256k1_scalar_cadd_bit(secp256k1_scalar *r, unsigned int bit, int
     if (flag && bit < 32)
         *r += ((uint32_t)1 << bit);
 #ifdef VERIFY
+    VERIFY_CHECK(bit < 32);
+    /* Verify that adding (1 << bit) will not overflow any in-range scalar *r by overflowing the underlying uint32_t. */
+    VERIFY_CHECK(((uint32_t)1 << bit) - 1 <= UINT32_MAX - EXHAUSTIVE_TEST_ORDER);
     VERIFY_CHECK(secp256k1_scalar_check_overflow(r) == 0);
 #endif
 }