Use only preallocated memory in ngx_readv_chain() (ticket #1408).

In d1bde5c3c5d2, the number of preallocated iovec's for ngx_readv_chain()
was increased.  Still, in some setups, the function might allocate memory
for iovec's from a connection pool, which is only freed when closing the
connection.

The ngx_readv_chain() function was modified to use only preallocated
memory, similarly to the ngx_writev_chain() change in 8e903522c17a.

src/os/unix/ngx_readv_chain.c

@@ -96,7 +96,7 @@ ngx_readv_chain(ngx_connection_t *c, ngx_chain_t *chain, off_t limit)
             iov->iov_len += n;
 
         } else {
-            if (vec.nelts >= IOV_MAX) {
+            if (vec.nelts == vec.nalloc) {
                 break;
             }