Fixed undefined behavior when left shifting signed integer.

src/os/unix/ngx_process_cycle.c

diff --git a/src/os/unix/ngx_process_cycle.c b/src/os/unix/ngx_process_cycle.c
index 7cee1c5..8c2d968 100644
--- a/src/os/unix/ngx_process_cycle.c
+++ b/src/os/unix/ngx_process_cycle.c
@@ -884,7 +884,7 @@
                       "sigprocmask() failed");
     }
 
-    srandom((ngx_pid << 16) ^ ngx_time());
+    srandom(((unsigned) ngx_pid << 16) ^ ngx_time());
 
     /*
      * disable deleting previous events for the listening sockets because