Fix memory error in networking unittest (#1697)

Fix new unittest networking use-after-free error


```
==96611==ERROR: AddressSanitizer: heap-use-after-free on address 0x503000075e00 at pc 0x55e52cbe1495 bp 0x7ffd9e1fc690 sp 0x7ffd9e1fc688
READ of size 8 at 0x503000075e00 thread T0
    #0 0x55e52cbe[149](https://github.com/valkey-io/valkey/actions/runs/13230922385/job/36927929457#step:10:150)4 in freeReplicaReferencedReplBuffer /home/runner/work/valkey/valkey/src/replication.c:401:27
    #1 0x55e52cbe7abf in freeClientReplicationData /home/runner/work/valkey/valkey/src/replication.c:1261:5
    #2 0x55e52cb17a44 in test_writeToReplica /home/runner/work/valkey/valkey/src/unit/test_networking.c:188:5
    #3 0x55e52cac976b in runTestSuite /home/runner/work/valkey/valkey/src/unit/test_main.c:26:28
    #4 0x55e52cac9bae in main /home/runner/work/valkey/valkey/src/unit/test_main.c:61:14
    #5 0x7fded4c2a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #6 0x7fded4c2a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #7 0x55e52c9b5ec4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x177ec4) (BuildId: 587aaf0e86abaf104cbb714f290b1436f8ddf614)

0x503000075e00 is located 16 bytes inside of 24-byte region [0x503000075df0,0x503000075e08)
freed by thread T0 here:
    #0 0x55e52ca50a7a in free (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x212a7a) (BuildId: 587aaf0e86abaf104cbb714f290b1436f8ddf614)
    #1 0x55e52cb905ba in listEmpty /home/runner/work/valkey/valkey/src/adlist.c:64:9
    #2 0x55e52cb179e5 in test_writeToReplica /home/runner/work/valkey/valkey/src/unit/test_networking.c:179:9
    #3 0x55e52cac976b in runTestSuite /home/runner/work/valkey/valkey/src/unit/test_main.c:26:28
    #4 0x55e52cac9bae in main /home/runner/work/valkey/valkey/src/unit/test_main.c:61:14
    #5 0x7fded4c2a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #6 0x7fded4c2a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #7 0x55e52c9b5ec4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x177ec4) (BuildId: 587aaf0e86abaf104cbb714f290b1436f8ddf614)

previously allocated by thread T0 here:
    #0 0x55e52ca50d13 in malloc (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x212d13) (BuildId: 587aaf0e86abaf104cbb714f290b1436f8ddf614)
    #1 0x55e52cbb844f in ztrymalloc_usable_internal /home/runner/work/valkey/valkey/src/zmalloc.c:[155](https://github.com/valkey-io/valkey/actions/runs/13230922385/job/36927929457#step:10:156):17
    #2 0x55e52cbb844f in valkey_malloc /home/runner/work/valkey/valkey/src/zmalloc.c:184:17
    #3 0x55e52cb90be6 in listAddNodeTail /home/runner/work/valkey/valkey/src/adlist.c:126:17
    #4 0x55e52cb17873 in test_writeToReplica /home/runner/work/valkey/valkey/src/unit/test_networking.c:167:9
    #5 0x55e52cac976b in runTestSuite /home/runner/work/valkey/valkey/src/unit/test_main.c:26:28
    #6 0x55e52cac9bae in main /home/runner/work/valkey/valkey/src/unit/test_main.c:61:14
    #7 0x7fded4c2a1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #8 0x7fded4c2a28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #9 0x55e52c9b5ec4 in _start (/home/runner/work/valkey/valkey/src/valkey-unit-tests+0x177ec4) (BuildId: 587aaf0e86abaf104cbb714f290b1436f8ddf614)
```


https://github.com/valkey-io/valkey/actions/runs/13230922385/job/36927929457

Signed-off-by: Uri Yagelnik <[email protected]>

src/unit/test_networking.c

@@ -180,6 +180,7 @@ int test_writeToReplica(int argc, char **argv, int flags) {
         zfree(fake_conn->buffer);
         zfree(fake_conn);
         zfree(block);
+        c->repl_data->ref_repl_buf_node = NULL;
     }
 
     /* Cleanup */
@@ -294,6 +295,7 @@ int test_postWriteToReplica(int argc, char **argv, int flags) {
 
         /* Cleanup */
         zfree(block);
+        c->repl_data->ref_repl_buf_node = NULL;
         listEmpty(server.repl_buffer_blocks);
     }