Core: added ngx_slab_calloc() and ngx_slab_calloc_locked().

These functions return zeroed memory, analogous to ngx_pcalloc().

src/core/ngx_slab.c

@@ -398,6 +398,35 @@ done:
 }
 
 
+void *
+ngx_slab_calloc(ngx_slab_pool_t *pool, size_t size)
+{
+    void  *p;
+
+    ngx_shmtx_lock(&pool->mutex);
+
+    p = ngx_slab_calloc_locked(pool, size);
+
+    ngx_shmtx_unlock(&pool->mutex);
+
+    return p;
+}
+
+
+void *
+ngx_slab_calloc_locked(ngx_slab_pool_t *pool, size_t size)
+{
+    void  *p;
+
+    p = ngx_slab_alloc_locked(pool, size);
+    if (p) {
+        ngx_memzero(p, size);
+    }
+
+    return p;
+}
+
+
 void
 ngx_slab_free(ngx_slab_pool_t *pool, void *p)
 {