Fix improper use of memset.

For the following code snippet -
  char *str;
  str = (char *)malloc(...);
  memset(str, '\0', sizeof(str));
                    ^^^^^^^^^^^ always equals 4 or 8

TEST=Built.
BUG=None

Change-Id: I42fb6c27a2bd8cdbeb4941d8b01e242132132154
Reviewed-on: https://gerrit.chromium.org/gerrit/47191
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Tested-by: Han Shen <shenhan@google.com>
Commit-Queue: Han Shen <shenhan@google.com>