Fixed overflow if ngx_slab_alloc() is called with very big "size" argument.

src/core/ngx_slab.c

diff --git a/src/core/ngx_slab.c b/src/core/ngx_slab.c
index 782792d..ae9d6f3 100644
--- a/src/core/ngx_slab.c
+++ b/src/core/ngx_slab.c
@@ -162,8 +162,8 @@
         ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, ngx_cycle->log, 0,
                        "slab alloc: %uz", size);
 
-        page = ngx_slab_alloc_pages(pool, (size + ngx_pagesize - 1)
-                                          >> ngx_pagesize_shift);
+        page = ngx_slab_alloc_pages(pool, (size >> ngx_pagesize_shift)
+                                          + ((size % ngx_pagesize) ? 1 : 0));
         if (page) {
             p = (page - pool->pages) << ngx_pagesize_shift;
             p += (uintptr_t) pool->start;