net: Reject + sign when parsing subnet mask

It does not make sense and it is rejected by other parsers as well:

>>> ipaddress.ip_network("1.2.3.0/+24")
ValueError: '1.2.3.0/+24' does not appear to be an IPv4 or IPv6 network

src/netbase.cpp

@@ -829,10 +829,9 @@ CSubNet LookupSubNet(const std::string& subnet_str)
         addr = static_cast<CNetAddr>(MaybeFlipIPv6toCJDNS(CService{addr.value(), /*port=*/0}));
         if (slash_pos != subnet_str.npos) {
             const std::string netmask_str{subnet_str.substr(slash_pos + 1)};
-            uint8_t netmask;
+            if (const auto netmask{ToIntegral<uint8_t>(netmask_str)}) {
-            if (ParseUInt8(netmask_str, &netmask)) {
                 // Valid number; assume CIDR variable-length subnet masking.
-                subnet = CSubNet{addr.value(), netmask};
+                subnet = CSubNet{addr.value(), *netmask};
             } else {
                 // Invalid number; try full netmask syntax. Never allow lookup for netmask.
                 const std::optional<CNetAddr> full_netmask{LookupHost(netmask_str, /*fAllowLookup=*/false)};

src/test/netbase_tests.cpp

@@ -150,7 +150,6 @@ BOOST_AUTO_TEST_CASE(embedded_test)
 
 BOOST_AUTO_TEST_CASE(subnet_test)
 {
-
     BOOST_CHECK(LookupSubNet("1.2.3.0/24") == LookupSubNet("1.2.3.0/255.255.255.0"));
     BOOST_CHECK(LookupSubNet("1.2.3.0/24") != LookupSubNet("1.2.4.0/255.255.255.0"));
     BOOST_CHECK(LookupSubNet("1.2.3.0/24").Match(ResolveIP("1.2.3.4")));
@@ -185,6 +184,7 @@ BOOST_AUTO_TEST_CASE(subnet_test)
     // Check valid/invalid
     BOOST_CHECK(LookupSubNet("1.2.3.0/0").IsValid());
     BOOST_CHECK(!LookupSubNet("1.2.3.0/-1").IsValid());
+    BOOST_CHECK(!LookupSubNet("1.2.3.0/+24").IsValid());
     BOOST_CHECK(LookupSubNet("1.2.3.0/32").IsValid());
     BOOST_CHECK(!LookupSubNet("1.2.3.0/33").IsValid());
     BOOST_CHECK(!LookupSubNet("1.2.3.0/300").IsValid());