(mop_up): Set work->previous_argument to NULL after freeing it.

From-SVN: r24663

libiberty/cplus-dem.c

 /* Demangler for GNU C++
-   Copyright 1989, 91, 94, 95, 96, 97, 1998 Free Software Foundation, Inc.
+   Copyright 1989, 91, 94, 95, 96, 97, 98, 1999 Free Software Foundation, Inc.
    Written by James Clark (jjc@jclark.uucp)
    Rewritten by Fred Fish (fnf@cygnus.com) for ARM and Lucid demangling
    Modified by Satish Pai (pai@apollo.hp.com) for HP demangling
@@ -875,6 +875,7 @@ mop_up (work, declp, success)
     {
       string_delete (work->previous_argument);
       free ((char*) work->previous_argument);
+      work->previous_argument = NULL;
     }
 
   /* If demangling was successful, ensure that the demangled string is null
@@ -4416,4 +4417,3 @@ xrealloc (ptr, size)
   return value;
 }
 #endif	/* main */
-