Are there any places where you're performing strcpy() or strcat() with that variable into another one (which you might not have expanded in size)? Are there any places where sprintf() is used?
As far as I recall, in DOS the "Null pointer assigment" error is reported when a particular memory area seems corrupt at the time a program exits. Part of the problem is DOS generally doesn't have memory protection (to prevent processes to modifying memory outside the defined boundaries) that would trigger an exception trap (on many Unix systems, you'll get "Bus error" error and a core dump at the point of the failure).