Fixed a dead store.

Found by Clang Static Analyzer.

src/nxt_random.c

@@ -58,8 +58,6 @@ nxt_random_stir(nxt_random_t *r)
         u_char      bytes[NXT_RANDOM_KEY_SIZE];
     } key;
 
-    n = 0;
-
 #if (NXT_HAVE_GETRANDOM)
 
     n = getrandom(&key, NXT_RANDOM_KEY_SIZE, 0);
@@ -70,6 +68,10 @@ nxt_random_stir(nxt_random_t *r)
 
     n = syscall(SYS_getrandom, &key, NXT_RANDOM_KEY_SIZE, 0);
 
+#else
+
+    n = 0;
+
 #endif
 
     if (n != NXT_RANDOM_KEY_SIZE) {