Fixes an error when reading `empty' cache files.

It uses the basic idea of 2008-12-27T17:26:06Z!kailoran@gmail.com, but the test added there
doesn't seem to catch all cases. Fixes bug #16544.

changelog

@@ -270,6 +270,7 @@ Version 1.11.0-svn:
      added to game description
    * Fixed bug #19322: Empty sides not being listed at the status table
    * Fixed bug #19681: Use the created cache instead of rebuilding it all the time
+   * Fixed bug #16544: Fail to read `empty' cache files.
    * Fix bug #19681: Cache is never valid
 
 Version 1.10.0:

src/serialization/parser.cpp

@@ -391,6 +391,15 @@ void read_gz(config &cfg, std::istream &file, abstract_validator * validator)
 	filter.push(boost::iostreams::gzip_decompressor());
 	filter.push(file);
 
+	/*
+	 * It sometimes seems the file is not empty but still no real data.
+	 * Filter that case here. It might be previous test is no longer required
+	 * but simply keep it.
+	 */
+	if(filter.peek() == EOF) {
+		return;
+	}
+
 	parser(cfg, filter,validator)();
 }