Fixed NULL assignment to ZString.

ZString.cpp

@@ -33,8 +33,18 @@ namespace coreutils {
       cursor = data;
    }
 
-   ZString::ZString(const char *data) : data((char *)data), length(strlen(data)), cursor((char *)data) {}
-
+   ZString::ZString(const char *data) {
+      if((char *)data != NULL) {	   
+	 this->data = (char *)data;
+	 this->length = strlen(data);
+	 this->cursor = (char *)data;
+      } else {
+	 this->data = NULL;
+	 this->length = 0;
+	 this->cursor = NULL;
+      }
+   }
+   
    ZString::ZString(char *data, size_t length) : data(data), length(length), cursor(data) {}
 
    ZString::ZString(unsigned char *data, size_t length) : data((char *)data), length(length), cursor((char *)data) {}