SUPERNOVA: Fix reading of .dat file

When searching for the right block in supernova.dat, the engine
didn't take into account the full size of each block (it thought,
it read 12 bytes less in each block) and thanks to that it
tryed to read one additional block at the end.

engines/supernova/supernova.cpp

@@ -501,7 +501,8 @@ Common::SeekableReadStream *SupernovaEngine::getBlockFromDatFile(Common::String
 			return f.readStream(size);
 		} else {
 			f.skip(size);
-			readSize += size;
+			// size + 4 bytes for id + 4 bytes for lang + 4 bytes for size
+			readSize += size + 12;
 		}
 	}