fixed cursor destruction on getLineNumberAtCursor() method to ZString.

ZString.cpp

@@ -576,9 +576,10 @@ namespace coreutils {
    }
 
    int ZString::getLineNumberAtCursor() {
+      char *temp = cursor;
       int line = 1;
-      while(cursor != data) 
-	if(*cursor-- == '\n')
+      while(temp != data) 
+	if(*temp-- == '\n')
 	  ++line;
       return line;      	   
    }