bot_update: Initialize synced_solutions before syncing.

If sync fails, synced_solutions is not initialized, resulting in a
use-before-init error.

Bug: 1217352
Change-Id: I2367d0f8030faa571b5c2743d5d97aa4c0329a7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2945663
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -1064,6 +1064,7 @@ def checkout(options, git_slns, specs, revisions, step_text):
     pass
 
   should_delete_dirty_file = False
+  synced_solutions = []
   experiments = []
   if options.experiments:
     experiments = options.experiments.split(',')