Fixed leak in MString when assigning from another MString.

testing/mstring_test.cpp

@@ -54,9 +54,10 @@ int main(int argc, char **argv) {
    // Reassignments.
 
    coreutils::MString test10;
+   std::cout << ">" << test10 << "<" << std::endl;
    test10 = "";
-   std::cout << test10 << std::endl;
-   
+   std::cout << ">" << test10 << "<" << std::endl;
+ 
    return 0;
 }