MINOR: debug: enable memory poisonning to use byte 0

When debugging an issue, sometimes it can be useful to be able to use
byte 0 to poison memory areas, resulting in the same effect as a calloc().
This patch changes the default mem_poison_byte to -1 to disable it so that
all positive values are usable.

include/common/debug.h

@@ -62,7 +62,7 @@
 static inline void *p_malloc(size_t size)
 {
 	void *ret = malloc(size);
-	if (mem_poison_byte && ret)
+	if (mem_poison_byte >= 0 && ret)
 		memset(ret, mem_poison_byte, size);
 	return ret;
 }

include/common/memory.h

@@ -43,8 +43,8 @@ struct pool_head {
 	char name[12];		/* name of the pool */
 };
 
-/* poison each newly allocated area with this byte if not null */
-extern char mem_poison_byte;
+/* poison each newly allocated area with this byte if >= 0 */
+extern int mem_poison_byte;
 
 /*
  * This function destroys a pull by freeing it completely.
@@ -141,7 +141,7 @@ static inline void *pool_alloc2(struct pool_head *pool)
 	void *p;
 
 	p = pool_alloc_dirty(pool);
-	if (p && mem_poison_byte)
+	if (p && mem_poison_byte >= 0)
 		memset(p, mem_poison_byte, pool->size);
 	return p;
 }

src/memory.c

@@ -20,7 +20,7 @@
 #include <proto/log.h>
 
 static struct list pools = LIST_HEAD_INIT(pools);
-char mem_poison_byte = 0;
+int mem_poison_byte = -1;
 
 /* Try to find an existing shared pool with the same characteristics and
  * returns it, otherwise creates this one. NULL is returned if no memory