Silence GCC 7 warning "control reaches end of non-void function" (-Wreturn-type) in psbt.cpp

src/psbt.cpp

@@ -337,7 +337,9 @@ std::string PSBTRoleName(PSBTRole role) {
     case PSBTRole::SIGNER: return "signer";
     case PSBTRole::FINALIZER: return "finalizer";
     case PSBTRole::EXTRACTOR: return "extractor";
+        // no default case, so the compiler can warn about missing cases
     }
+    assert(false);
 }
 
 bool DecodeBase64PSBT(PartiallySignedTransaction& psbt, const std::string& base64_tx, std::string& error)