Fix asan failures.

src/core/ext/client_config/subchannel.h

@@ -163,7 +163,7 @@ struct grpc_subchannel_args {
   /** Channel arguments to be supplied to the newly created channel */
   const grpc_channel_args *args;
   /** Server name */
-  char *server_name;  // Does not own.
+  char *server_name;
   /** Address to connect to */
   struct sockaddr *addr;
   size_t addr_len;

src/core/ext/client_config/subchannel_index.c

@@ -38,6 +38,7 @@
 
 #include <grpc/support/alloc.h>
 #include <grpc/support/avl.h>
+#include <grpc/support/string_util.h>
 #include <grpc/support/tls.h>
 
 #include "src/core/lib/channel/channel_args.h"
@@ -85,7 +86,7 @@ static grpc_subchannel_key *create_key(
   } else {
     k->args.filters = NULL;
   }
-  k->args.server_name = args->server_name;
+  k->args.server_name = gpr_strdup(args->server_name);
   k->args.addr_len = args->addr_len;
   k->args.addr = gpr_malloc(args->addr_len);
   if (k->args.addr_len > 0) {
@@ -131,6 +132,7 @@ void grpc_subchannel_key_destroy(grpc_exec_ctx *exec_ctx,
   grpc_connector_unref(exec_ctx, k->connector);
   gpr_free((grpc_channel_args *)k->args.filters);
   grpc_channel_args_destroy((grpc_channel_args *)k->args.args);
+  gpr_free(k->args.server_name);
   gpr_free(k->args.addr);
   gpr_free(k);
 }