refactor: Fix CAddrMan::Check style

This change improves readability, and follows Developer Notes.

src/addrman.h

@@ -716,10 +716,9 @@ private:
     void Check()
     {
 #ifdef DEBUG_ADDRMAN
-        {
         LOCK(cs);
-            int err;
+        const int err = Check_();
-            if ((err=Check_()))
+        if (err) {
             LogPrintf("ADDRMAN CONSISTENCY CHECK FAILED!!! err=%i\n", err);
         }
 #endif