Increase robustness against UB.

Thanks to elichai2 who noted that the literal '1' is a signed integer, and that shifting a signed 32-bit integer by 31 bits causes an overflow and yields undefined behaviour.
While 'scalar_low_impl''s 'secp256k1_scalar_cadd_bit' is only used for testing purposes and currently the 'bit' parameter is only 0 or 1, it is better to avoid undefined behaviour in case the used domain of 'secp256k1_scalar_cadd_bit' expands.

src/scalar_low_impl.h

@@ -38,7 +38,7 @@ static int secp256k1_scalar_add(secp256k1_scalar *r, const secp256k1_scalar *a,
 
 static void secp256k1_scalar_cadd_bit(secp256k1_scalar *r, unsigned int bit, int flag) {
     if (flag && bit < 32)
-        *r += (1 << bit);
+        *r += ((uint32_t)1 << bit);
 #ifdef VERIFY
     VERIFY_CHECK(secp256k1_scalar_check_overflow(r) == 0);
 #endif