Frame Sets: Occasional lock ups

[nschlia]

The frame set checks occasionally lock up, causing the ffmpegfs process to stop responding. Access mutex race conditions are likely. The process does not terminate, the "make check" process stays forever.

The issue appears to have existed for some time, but it has become more visible now that others have been resolved.

Happens with test_frameset_* checks only, mostly with png.

test_frameset_png.log
test_frameset_png_builtin.log