Fix a crash (assert) when client set serial version < 24 in INIT

When SER_FMT_VER_LOWEST is set to zero, then the test is stupid in INIT because all client works. In mapblock we check if client's serialization version is < 24, but if client sent serialization version < 24 (15 for example) the server set it and tried to send nodes, then BOOM

To resolve the problem:
* Create a different CLIENT_MIN_VERSION to handle this problem
* Remove the exception
* Use an assert in case of bad developer code

src/mapblock.cpp

@@ -526,11 +526,7 @@ void MapBlock::serialize(std::ostream &os, u8 version, bool disk)
 		throw SerializationError("ERROR: Not writing dummy block.");
 	}
 
-	// Can't do this anymore; we have 16-bit dynamically allocated node IDs
-	// in memory; conversion just won't work in this direction.
-	if(version < 24)
-		throw SerializationError("MapBlock::serialize: serialization to "
-				"version < 24 not possible");
+	assert(version >= SER_FMT_CLIENT_VER_LOWEST);
 
 	// First byte
 	u8 flags = 0;

src/mapblock.h

@@ -494,6 +494,7 @@ public:
 	
 	// These don't write or read version by itself
 	// Set disk to true for on-disk format, false for over-the-network format
+	// Precondition: version >= SER_FMT_CLIENT_VER_LOWEST
 	void serialize(std::ostream &os, u8 version, bool disk);
 	// If disk == true: In addition to doing other things, will add
 	// unknown blocks from id-name mapping to wndef

src/serialization.h

@@ -71,6 +71,10 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #define SER_FMT_VER_HIGHEST_WRITE 25
 // Lowest supported serialization version
 #define SER_FMT_VER_LOWEST 0
+// Lowest client supported serialization version
+// Can't do < 24 anymore; we have 16-bit dynamically allocated node IDs
+// in memory; conversion just won't work in this direction.
+#define SER_FMT_CLIENT_VER_LOWEST 24
 
 inline bool ser_ver_supported(s32 v) {
 	return v >= SER_FMT_VER_LOWEST && v <= SER_FMT_VER_HIGHEST_READ;

src/server.cpp

@@ -1366,7 +1366,7 @@ void Server::ProcessData(u8 *data, u32 datasize, u16 peer_id)
 		// Use the highest version supported by both
 		int deployed = std::min(client_max, our_max);
 		// If it's lower than the lowest supported, give up.
-		if(deployed < SER_FMT_VER_LOWEST)
+		if(deployed < SER_FMT_CLIENT_VER_LOWEST)
 			deployed = SER_FMT_VER_INVALID;
 
 		if(deployed == SER_FMT_VER_INVALID)