malloc(0) now returns NULL with simple/standard allocator

Author: ZERICO2005

src/libc/allocator_simple.src

@@ -4,13 +4,15 @@
 
 	public	__simple_malloc
 __simple_malloc:
+	; returns NULL when size is zero
 	pop	bc
 	ex	(sp),hl
 	push	bc
 	ld	de,(_heap_ptr)
+	dec	hl
 	add	hl,de
 	jr	c,.null
-	ld	bc,___heaptop
+	ld	bc,___heaptop-1
 	sbc	hl,bc
 	jr	nc,.null
 	add	hl,bc

src/libc/allocator_standard.c

@@ -21,7 +21,8 @@ void *_standard_malloc(size_t alloc_size)
 
     /* add size of block header to real size */
     const size_t size = alloc_size + sizeof(block_t);
-    if (size < alloc_size)
+    /* abort if alloc_size is 0 or size overflowed */
+    if (size <= alloc_size)
     {
         return NULL;
     }