71200ac390 [fuzz] Only check duplicate coinbase script when block was valid (dergoegge)
Pull request description:
Partially revert #27780, because moving the duplicate coinbase check out of the `was_valid` branch leads to non-bug crashes in the fuzz target.
For context and further explanation see: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=59516
ACKs for top commit:
MarcoFalke:
nice lgtm ACK 71200ac390
Tree-SHA512: 8c38e5ff9de6331016b9a0c5e435d007d46186151b04c09085f617bb31627a28ad56678066fe152372a3ad8656f026439e3e2f9ee61d7ef588072aef8124eaa3