[Net] Fix miniupnpc when no interface is specified

This is a tricky one, it used to work, but it was wrong, because in such
a scenario instead of passing NULL as required by the API, it would pass
a buffer containing the `\0` terminator.
This stopped working on a specific miniupnpc version, when they fixed
some network endianess issue on Windows, to which we made a workaround,
which in turn would probably result in failures when the interface is
specified.

This commit address the issue properly, by checking the specified
interface string size, and correctly passing NULL instead of the empty
string when necessary.

Also reverts the commit that introduced the bogus workaround:
388adac947

One of those PR when the explanation is much longer then code changes
:).

modules/upnp/upnp.cpp

@@ -53,10 +53,12 @@ int UPNP::discover(int timeout, int ttl, const String &device_filter) {
 	int error = 0;
 	struct UPNPDev *devlist;
 
+	CharString cs = discover_multicast_if.utf8();
+	const char *m_if = cs.length() ? cs.get_data() : NULL;
 	if (is_common_device(device_filter)) {
-		devlist = upnpDiscover(timeout, discover_multicast_if.utf8().get_data(), NULL, discover_local_port, discover_ipv6, ttl, &error);
+		devlist = upnpDiscover(timeout, m_if, NULL, discover_local_port, discover_ipv6, ttl, &error);
 	} else {
-		devlist = upnpDiscoverAll(timeout, discover_multicast_if.utf8().get_data(), NULL, discover_local_port, discover_ipv6, ttl, &error);
+		devlist = upnpDiscoverAll(timeout, m_if, NULL, discover_local_port, discover_ipv6, ttl, &error);
 	}
 
 	if (error != UPNPDISCOVER_SUCCESS) {

thirdparty/README.md

@@ -292,7 +292,6 @@ Files extracted from upstream source:
 - All `*.c` and `*.h` files from `miniupnpc` to `thirdparty/miniupnpc/miniupnpc`
 - Remove `test*`, `minihttptestserver.c` and `wingenminiupnpcstrings.c`
 
-The patch `windows_fix.diff` is applied to `minissdpc.c` to fix an upstream issue.
 The only modified file is miniupnpcstrings.h, which was created for Godot
 (it is usually autogenerated by cmake).
 

thirdparty/miniupnpc/miniupnpc/minissdpc.c

@@ -683,7 +683,11 @@ ssdpDiscoverDevices(const char * const deviceTypes[],
 #endif
 		} else {
 			struct in_addr mc_if;
+#if defined(_WIN32) && (_WIN32_WINNT >= _WIN32_WINNT_VISTA)
+			InetPtonA(AF_INET, multicastif, &mc_if);
+#else
 			mc_if.s_addr = inet_addr(multicastif); /* ex: 192.168.x.x */
+#endif
 			if(mc_if.s_addr != INADDR_NONE)
 			{
 				((struct sockaddr_in *)&sockudp_r)->sin_addr.s_addr = mc_if.s_addr;

thirdparty/miniupnpc/windows_fix.diff

@@ -1,16 +0,0 @@
-diff --git a/thirdparty/miniupnpc/miniupnpc/minissdpc.c b/thirdparty/miniupnpc/miniupnpc/minissdpc.c
-index 29f8110155..ea9af02e1f 100644
---- a/thirdparty/miniupnpc/miniupnpc/minissdpc.c
-+++ b/thirdparty/miniupnpc/miniupnpc/minissdpc.c
-@@ -683,11 +683,7 @@ ssdpDiscoverDevices(const char * const deviceTypes[],
- #endif
- 		} else {
- 			struct in_addr mc_if;
--#if defined(_WIN32) && (_WIN32_WINNT >= _WIN32_WINNT_VISTA)
--			InetPtonA(AF_INET, multicastif, &mc_if);
--#else
- 			mc_if.s_addr = inet_addr(multicastif); /* ex: 192.168.x.x */
--#endif
- 			if(mc_if.s_addr != INADDR_NONE)
- 			{
- 				((struct sockaddr_in *)&sockudp_r)->sin_addr.s_addr = mc_if.s_addr;