values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    BOOST_CHECK(IsHexNumber("0x0"));
    BOOST_CHECK(IsHexNumber("0"));
    BOOST_CHECK(IsHexNumber("0x10"));
    BOOST_CHECK(IsHexNumber("10"));
    BOOST_CHECK(IsHexNumber("0xff"));
    BOOST_CHECK(IsHexNumber("ff"));
    BOOST_CHECK(IsHexNumber("0xFfa"));
    BOOST_CHECK(IsHexNumber("Ffa"));
    BOOST_CHECK(IsHexNumber("0x00112233445566778899aabbccddeeffAABBCCDDEEFF"));
    BOOST_CHECK(IsHexNumber("00112233445566778899aabbccddeeffAABBCCDDEEFF"));
    BOOST_CHECK(!IsHexNumber(""));   
    BOOST_CHECK(!IsHexNumber("0x")); 
    BOOST_CHECK(!IsHexNumber("0x0 ")); 
    BOOST_CHECK(!IsHexNumber(" 0x0")); 
    BOOST_CHECK(!IsHexNumber("0x 0")); 
    BOOST_CHECK(!IsHexNumber(" "));    
    BOOST_CHECK(!IsHexNumber("0x0ga")); 
    BOOST_CHECK(!IsHexNumber("x0"));    
    BOOST_CHECK(!IsHexNumber("0x0x00")); 
}
BOOST_AUTO_TEST_CASE(util_seed_insecure_rand)