upload_symbols: fix logic error with upload_count

Fix crash seen on buildbots when retrying:
10:58:13: ERROR: Traceback (most recent call last):
  File "/b/cbuild/peppy-canary-master/chromite/buildbot/builderstage.py", line 335, in Run
    self.PerformStage()
  File "/b/cbuild/peppy-canary-master/chromite/buildbot/cbuildbot_stages.py", line 3135, in PerformStage
    self.UploadSymbols(buildroot, board)
  File "/b/cbuild/peppy-canary-master/chromite/buildbot/cbuildbot_stages.py", line 3156, in UploadSymbols
    commands.UploadSymbols(buildroot, board, official, cnt)
  File "/b/cbuild/peppy-canary-master/chromite/buildbot/cbuildbot_commands.py", line 1246, in UploadSymbols
    root=os.path.join(buildroot, constants.DEFAULT_CHROOT_DIR))
  File "/b/cbuild/peppy-canary-master/chromite/scripts/upload_symbols.py", line 364, in UploadSymbols
    upload_count += len(sym_paths)
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'int'

BUG=None
TEST=`./scripts/upload_symbols_unittest.py` passes

Change-Id: I3b82e509cb9dce13389ef649b98109953e1dbda6
Reviewed-on: https://chromium-review.googlesource.com/180060
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/upload_symbols.py b/scripts/upload_symbols.py
index b999ae8..51a4947 100644
--- a/scripts/upload_symbols.py
+++ b/scripts/upload_symbols.py
@@ -361,7 +361,8 @@
       sym_paths.append(failed_queue.get())
     if sym_paths:
       cros_build_lib.Warning('retrying %i symbols', len(sym_paths))
-      upload_count += len(sym_paths)
+      if upload_count is not None:
+        upload_count += len(sym_paths)
       # Decrement the error count in case we recover in the second pass.
       assert bg_errors.value >= len(sym_paths), 'more failed files than errors?'
       bg_errors.value -= len(sym_paths)