Fix compiler error on need_to_unref_constructed

`need_to_unref_constructed` is not initialized, making Xcode compiler [complain on Sierra](https://sponge-qa.corp.google.com/invocation?tab=Kokoro&id=0ff33b71-2f61-4ad6-837e-5e43043c282a&searchFor=):
```
/Volumes/BuildData/tmpfs/src/github/grpc/workspace_objc_macos_dbg_native/src/core/ext/filters/client_channel/subchannel_index.cc:206:7: error: variable 'need_to_unref_constructed' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/core/ext/filters/client_channel/subchannel_index.cc b/src/core/ext/filters/client_channel/subchannel_index.cc
index ae39ca394e..1624643d0b 100644
--- a/src/core/ext/filters/client_channel/subchannel_index.cc
+++ b/src/core/ext/filters/client_channel/subchannel_index.cc
@@ -163,7 +163,7 @@ grpc_subchannel* grpc_subchannel_index_register(grpc_exec_ctx* exec_ctx,
                                                 grpc_subchannel_key* key,
                                                 grpc_subchannel* constructed) {
   grpc_subchannel* c = nullptr;
-  bool need_to_unref_constructed;
+  bool need_to_unref_constructed = false;
 
   while (c == nullptr) {
     need_to_unref_constructed = false;