Removed implicit call of nxt_thread() in nxt_thread_tid().

Explicit is better than implicit © The Zen of Python.

The nxt_thread_tid(NULL) call was used only twice in the code
and such behaviour was specific to nxt_thread_tid() function.

src/nxt_mp.c

@@ -233,7 +233,7 @@ nxt_mp_thread_adopt(nxt_mp_t *mp)
 {
 #if (NXT_DEBUG)
     mp->pid = nxt_pid;
-    mp->tid = nxt_thread_tid(NULL);
+    mp->tid = nxt_thread_tid(nxt_thread());
 #endif
 }
 

src/nxt_random.c

@@ -99,7 +99,7 @@ nxt_random_stir(nxt_random_t *r)
         key.value[0] ^= tv.tv_usec;
         key.value[1] ^= tv.tv_sec;
         key.value[2] ^= nxt_pid;
-        key.value[3] ^= (uintptr_t) nxt_thread_tid(NULL);
+        key.value[3] ^= (uintptr_t) nxt_thread_tid(nxt_thread());
     }
 
     nxt_random_add(r, key.bytes, NXT_RANDOM_KEY_SIZE);

src/nxt_thread.c

@@ -242,10 +242,6 @@ nxt_thread_wait(nxt_thread_handle_t handle)
 nxt_tid_t
 nxt_thread_tid(nxt_thread_t *thr)
 {
-    if (thr == NULL) {
-        thr = nxt_thread();
-    }
-
 #if (NXT_HAVE_THREAD_STORAGE_CLASS)
 
     if (nxt_slow_path(thr->tid == 0)) {