Fixed undefined behaviour in left shift of int value

Found by UndefinedBehaviorSanitizer:

src/nxt_random.c:151:31: runtime error: left shift of 140 by 24 places cannot be represented in type 'int'
    #0 0x104f78968 in nxt_random nxt_random.c:151
    #1 0x104f58a98 in nxt_shm_open nxt_port_memory.c:377
    #2 0x10503e24c in nxt_controller_conf_send nxt_controller.c:617
    #3 0x105041154 in nxt_controller_process_request nxt_controller.c:1109
    #4 0x104fcdc48 in nxt_event_engine_start nxt_event_engine.c:542
    #5 0x104f27254 in main nxt_main.c:35
    #6 0x180fbd0dc  (<unknown module>)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/nxt_random.c:151:31

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_random.c

@@ -148,10 +148,10 @@ nxt_random(nxt_random_t *r)
         nxt_random_stir(r);
     }
 
-    val  = nxt_random_byte(r) << 24;
-    val |= nxt_random_byte(r) << 16;
-    val |= nxt_random_byte(r) << 8;
-    val |= nxt_random_byte(r);
+    val  = (uint32_t) nxt_random_byte(r) << 24;
+    val |= (uint32_t) nxt_random_byte(r) << 16;
+    val |= (uint32_t) nxt_random_byte(r) << 8;
+    val |= (uint32_t) nxt_random_byte(r);
 
     return val;
 }