target/cortex_m: do not leak memory on reexamination

This bug was exposed by Valgrind.

Change-Id: If50878664d928c0a44e309ca1452089c1ac71466
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2542
Tested-by: jenkins
Reviewed-by: Stian Skjelstad <stian@nixia.no>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/register.c

@@ -72,6 +72,14 @@ struct reg_cache **register_get_last_cache_p(struct reg_cache **first)
 	return cache_p;
 }
 
+void register_unlink_cache(struct reg_cache **cache_p, const struct reg_cache *cache)
+{
+	while (*cache_p && *cache_p != cache)
+		cache_p = &((*cache_p)->next);
+	if (*cache_p)
+		*cache_p = cache->next;
+}
+
 /** Marks the contents of the register cache as invalid (and clean). */
 void register_cache_invalidate(struct reg_cache *cache)
 {