Do not warn about primitives of size 0.

Otherwise this shows a warning for every RPG Maker 2003 English database which contains a 0-byte version field.

This issue always existed but was hidden behind LCF_DEBUG_TRACE.

src/reader_struct.h

@@ -268,12 +268,12 @@ struct Primitive<int32_t> {
 #ifdef LCF_DEBUG_TRACE
 			fprintf(stderr, "  %d\n", ref);
 #endif
-		} else {
+		} else if (length == 0) {
 			ref = 0;
+		} else {
 			Log::Warning("Invalid integer at %X", stream.Tell());
 			stream.Seek(length, LcfReader::FromCurrent);
 		}
-
 	}
 	static void WriteLcf(const int32_t& ref, LcfWriter& stream) {
 		stream.WriteInt(ref);