Fix clang-tidy readability-isolate-declaration warnings

I missed one of these in review so a machine is probably better at
catching them.

I fixed the existing instances which where largely in code which look
dated. Where possible I slightly reorganized the code so we do not have
to leave values uninitialized, but did not touch up anything else.

src/DNS_Mapping.cc

@@ -43,7 +43,8 @@ DNS_Mapping::DNS_Mapping(FILE* f) {
         return;
     }
 
-    char req_buf[512 + 1], name_buf[512 + 1];
+    char req_buf[512 + 1];
+    char name_buf[512 + 1];
     int is_req_host;
     int failed_local;
     int num_addrs;