buflib: Add crc field protecting buflib cookie integrity

This should catch the case of buffer misuse which results
in corrupted cookie of next allocation. The check is performed
on move_block() so it may be a bit late.

There is buflib_check_valid() provided which checks the
integrity of all cookies for given context.
On DEBUG build with --sdl-thread this check is carried out
for core_ctx on every context switch to catch problems earlier.

Change-Id: I999d4576084592394e3dbd3bdf0f32935ff5f601
Reviewed-on: http://gerrit.rockbox.org/711
Reviewed-by: Thomas Martitz <kugel@rockbox.org>

firmware/thread.c

@@ -39,6 +39,7 @@
 #ifdef RB_PROFILE
 #include <profile.h>
 #endif
+#include "core_alloc.h"
 #include "gcc_extensions.h"
 
 /****************************************************************************
@@ -1161,6 +1162,11 @@ void switch_thread(void)
      * to this call. */
     store_context(&thread->context);
 
+#ifdef DEBUG
+    /* Check core_ctx buflib integrity */
+    core_check_valid();
+#endif
+
     /* Check if the current thread stack is overflown */
     if (UNLIKELY(thread->stack[0] != DEADBEEF) && thread->stack_size > 0)
         thread_stkov(thread);