Don't serialize StaticObjectList with > 65535 objects

Because the count is serialized as u16, this would cause overflow.

If minetest later deserialized a mapblock with an incorrect
static object count, it would be unable to find the NameIdMapping
(which comes after the StaticObjectList) and abort with an error
such as "Invalid block data in database: unsupported NameIdMapping
version" (issue #2610).

src/staticobject.cpp

@@ -19,6 +19,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #include "staticobject.h"
 #include "util/serialize.h"
+#include "log.h"
 
 void StaticObject::serialize(std::ostream &os)
 {
@@ -44,9 +45,20 @@ void StaticObjectList::serialize(std::ostream &os)
 	// version
 	u8 version = 0;
 	writeU8(os, version);
+
 	// count
-	u16 count = m_stored.size() + m_active.size();
+	size_t count = m_stored.size() + m_active.size();
+	// Make sure it fits into u16, else it would get truncated and cause e.g.
+	// issue #2610 (Invalid block data in database: unsupported NameIdMapping version).
+	if (count > (u16)-1) {
+		errorstream << "StaticObjectList::serialize(): "
+			<< "too many objects (" << count << ") in list, "
+			<< "not writing them to disk." << std::endl;
+		writeU16(os, 0);  // count = 0
+		return;
+	}
 	writeU16(os, count);
+
 	for(std::vector<StaticObject>::iterator
 			i = m_stored.begin();
 			i != m_stored.end(); ++i) {