[intel] Assert against 0-sized buffers in dri_bufmgr_fake.c.

They shouldn't be created, and this often helps catch stupid issues.

src/mesa/drivers/dri/common/dri_bufmgr_fake.c

@@ -554,6 +554,8 @@ dri_fake_bo_alloc(dri_bufmgr *bufmgr, const char *name,
 
    bufmgr_fake = (dri_bufmgr_fake *)bufmgr;
 
+   assert(size != 0);
+
    bo_fake = calloc(1, sizeof(*bo_fake));
    if (!bo_fake)
       return NULL;
@@ -590,6 +592,8 @@ dri_fake_bo_alloc_static(dri_bufmgr *bufmgr, const char *name,
 
    bufmgr_fake = (dri_bufmgr_fake *)bufmgr;
 
+   assert(size != 0);
+
    bo_fake = calloc(1, sizeof(*bo_fake));
    if (!bo_fake)
       return NULL;