Make FREE() clear pointer after memory released

Since a pointer to allocated memory mustn't be used after the memory is
freed, it is safer to clear the pointer. It also means that if the pointer
is subsequently used, it shoud segfault immediately rather than potentially
trampling over random memory, which might be very difficult to debug.

Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>

lib/memory.h

@@ -45,7 +45,8 @@ extern void *zalloc(unsigned long size);
 #define MALLOC(n)    ( keepalived_malloc((n), \
 		      (__FILE__), (char *)(__FUNCTION__), (__LINE__)) )
 #define FREE(b)      ( keepalived_free((b), \
-		      (__FILE__), (char *)(__FUNCTION__), (__LINE__)) )
+		      (__FILE__), (char *)(__FUNCTION__), (__LINE__)), \
+		       (b) = NULL )
 #define REALLOC(b,n) ( keepalived_realloc((b), (n), \
 		      (__FILE__), (char *)(__FUNCTION__), (__LINE__)) )
 
@@ -60,12 +61,12 @@ extern void keepalived_free_final(char *);
 #else
 
 #define MALLOC(n)    (zalloc(n))
-#define FREE(p)      (free(p))
+#define FREE(p)      (free(p), (p) = NULL)
 #define REALLOC(p,n) (realloc((p),(n)))
 
 #endif
 
 /* Common defines */
-#define FREE_PTR(P) if((P)) FREE((P)), (P) = NULL;
+#define FREE_PTR(P) if((P)) FREE((P));
 
 #endif