adb: don't append a slash to a path that already ends with one.

Bug: http://b/26964908
Change-Id: I1fbd752fd97f5414a306b46f7bfce3373362f8b3
diff --git a/file_sync_client.cpp b/file_sync_client.cpp
index 0fa5917..51fc143 100644
--- a/file_sync_client.cpp
+++ b/file_sync_client.cpp
@@ -753,8 +753,11 @@
         if (dst_isdir) {
             // If we're copying a local file to a remote directory,
             // we really want to copy to remote_dir + "/" + local_filename.
-            path_holder = android::base::StringPrintf(
-                "%s/%s", dst_path, adb_basename(src_path).c_str());
+            path_holder = dst_path;
+            if (path_holder.back() != '/') {
+                path_holder.push_back('/');
+            }
+            path_holder += adb_basename(src_path);
             dst_path = path_holder.c_str();
         }
         sc.SetExpectedTotalBytes(st.st_size);