refactor: Revert addition of univalue sighash string check

This check is already done by the rpc parser. Re-doing it is adding dead
code. Instead, throwing an exception when the assumption does not hold
is the already correct behavior.

To make the fuzz test more accurate and not swallow all runtime errors,
add a check that the passed in UniValue sighash argument is either a
string or null.

Co-authored-by: stickies-v <stickies-v@protonmail.com>

src/rpc/util.cpp

@@ -313,14 +313,16 @@ UniValue DescribeAddress(const CTxDestination& dest)
     return std::visit(DescribeAddressVisitor(), dest);
 }
 
+/**
+ * Returns a sighash value corresponding to the passed in argument.
+ *
+ * @pre The sighash argument should be string or null.
+*/
 int ParseSighashString(const UniValue& sighash)
 {
     if (sighash.isNull()) {
         return SIGHASH_DEFAULT;
     }
-    if (!sighash.isStr()) {
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "sighash needs to be null or string");
-    }
     const auto result{SighashFromStr(sighash.get_str())};
     if (!result) {
         throw JSONRPCError(RPC_INVALID_PARAMETER, util::ErrorString(result).original);

src/test/fuzz/parse_univalue.cpp

@@ -67,7 +67,7 @@ FUZZ_TARGET(parse_univalue, .init = initialize_parse_univalue)
     } catch (const std::runtime_error&) {
     }
     try {
-        (void)ParseSighashString(univalue);
+        if (univalue.isNull() || univalue.isStr()) (void)ParseSighashString(univalue);
     } catch (const UniValue&) {
     }
     try {