fio_test: only add to error count if nonzero error code

_error error code will be 0 when no errors are found. Make sure
that we do not increase _fail_count if we see an error code 0.

Fixup: crrev.com/c/1954578

BUG=chromium:1040166
TEST=Ran hardware_StorageFio

Change-Id: I6750ced168a3cd6f494e1b8bbc12445936ed9039
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/1992408
Tested-by: Alexis Savery <asavery@chromium.org>
Commit-Queue: Alexis Savery <asavery@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
diff --git a/client/cros/storage_tests/fio_test.py b/client/cros/storage_tests/fio_test.py
index a4ad98e..c1f7171 100644
--- a/client/cros/storage_tests/fio_test.py
+++ b/client/cros/storage_tests/fio_test.py
@@ -213,7 +213,7 @@
         for k, v in results.iteritems():
             if k.endswith('_error'):
                 self._error_code = int(v)
-                if self._fail_count == 0:
+                if self._error_code != 0 and self._fail_count == 0:
                     self._fail_count = 1
             elif k.endswith('_total_err'):
                 self._fail_count = int(v)