When searching for the active NLA strip, avoid overwriting the found strip
pointer with NULL if it was already found in a previous iteration.
The active strip is searched for while looping over the NLA tracks. If the
active strip was found on a previous track, and not on the current track,
this would effectively set `actstrip = NULL`. This is now avoided.
Another benefit is that the search for the active strip is stopped as soon
as it's found, which should increase performance a tiny bit.
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