Slab: improved double free detection.

Previously, an attempt to double free the starting page of the
free range was not detected.

src/core/ngx_slab.c

diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
index b2adbf4..0fc63b5 100644
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -597,7 +597,7 @@
             goto wrong_chunk;
         }
 
-        if (slab == NGX_SLAB_PAGE_FREE) {
+        if (!(slab & NGX_SLAB_PAGE_START)) {
             ngx_slab_error(pool, NGX_LOG_ALERT,
                            "ngx_slab_free(): page is already free");
             goto fail;