Fixed NULL assignment to ZString.

testing/zstring_test.cpp

@@ -84,7 +84,7 @@ int main(int argc, char **argv) {
 
    coreutils::ZString test14("25.5");
    coreutils::ZString test15("xyx");
-   std::cout << test14.startsWithDouble() << ":" << test15.startsWithDouble() << std::endl;
+   std::cout << test14.startsWithNumber() << ":" << test15.startsWithNumber() << std::endl;
 
    coreutils::ZString test16("This is a test of substring");
    std::cout << test16.substring(10) << std::endl;
@@ -102,5 +102,8 @@ int main(int argc, char **argv) {
 
    coreutils::ZString test19("1,");
    std::cout << test19.asDouble() << ":" << test19.unparsed() << std::endl;
+
+   std::cout << "Null assignment" << std::endl;
+   coreutils::ZString test20(NULL);
    
 }