refactor: Replace const char* to std::string

Some functions should be returning std::string instead of const char*.
This commit changes that.

src/script/standard.cpp

@@ -9,6 +9,8 @@
 #include <pubkey.h>
 #include <script/script.h>
 
+#include <string>
+
 typedef std::vector<unsigned char> valtype;
 
 bool fAcceptDatacarrier = DEFAULT_ACCEPT_DATACARRIER;
@@ -25,7 +27,7 @@ WitnessV0ScriptHash::WitnessV0ScriptHash(const CScript& in)
     CSHA256().Write(in.data(), in.size()).Finalize(begin());
 }
 
-const char* GetTxnOutputType(txnouttype t)
+std::string GetTxnOutputType(txnouttype t)
 {
     switch (t)
     {
@@ -39,7 +41,7 @@ const char* GetTxnOutputType(txnouttype t)
     case TX_WITNESS_V0_SCRIPTHASH: return "witness_v0_scripthash";
     case TX_WITNESS_UNKNOWN: return "witness_unknown";
     }
-    return nullptr;
+    assert(false);
 }
 
 static bool MatchPayToPubkey(const CScript& script, valtype& pubkey)