fuzz: add SelectParams to rbf.cpp

We cannot fuzz RBF (or do anything mempool-related, really) without
chainparams. This has been true since 2019 at least. I suspect this fuzz
test has never really been run.

src/test/fuzz/rbf.cpp

@@ -15,7 +15,13 @@
 #include <string>
 #include <vector>
 
-FUZZ_TARGET(rbf)
+void initialize_rbf(void) {
+    // ELEMENTS: our mempool needs Params() to be set for multiple reasons -- to check
+    //  the discount CT rate, to figure out pegin policy, etc
+    SelectParams(CBaseChainParams::LIQUID1);
+}
+
+FUZZ_TARGET_INIT(rbf, initialize_rbf)
 {
     FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
     SetMockTime(ConsumeTime(fuzzed_data_provider));