device: check for NULL driver in add_port_driver

This patch fixes a crash in MMDevice::add_port_driver() due to
g_str_equal() dereferencing a NULL driver returned by
mm_device_utils_get_port_driver().

Bug reported on https://code.google.com/p/chromium/issues/detail?id=241823
(cherry picked from commit 82a0787906dbf06219202345e2d39be1cb4e9350)

BUG=chromium:241823
TEST=Run network3g suites.

Change-Id: I14f7ed92b455dc2dc00b03ce5ac8860ad60706ba
Reviewed-on: https://gerrit.chromium.org/gerrit/56660
Commit-Queue: Ben Chan <benchan@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
1 file changed