rcxml: fix broken OSD layout with multiple <fields> entries

rcxml: fix broken OSD layout with multiple <fields> entries

Before this commit, all <field> entries inside different <fields> entires
were inserted to the same list. Suppose we have following configuration:

  <windowSwitcher>
    <fields><field content="title" width="100%" /></fields>
  </windowSwitcher>
  <windowSwitcher>
    <fields><field content="identifier" width="100%" /></fields>
  </windowSwitcher>

In this case, both two <field> entries were inserted to
rc.window_switcher.fields, making the OSD content overflow.

This commit fixes by clearing rc.window_switcher.fields when the parser
encounters <windowSwitcher><fields>.