removed some surplus null appends that got typoed in 0e29f7974b

A couple of null appends got typoed to append a '0' character instead.
Removed them here since String already takes care of the null terminator for us.

core/ustring.cpp

@@ -3881,8 +3881,6 @@ String String::percent_decode() const {
 		pe += c;
 	}
 
-	pe += '0';
-
 	return String::utf8(pe.ptr());
 }