avfilter: take_samples: do not directly return frame when samples are skipped

Modifying data pointer when skipping samples may make it unaligned.
Workaround for Ticket6349.

This should fix the crash of ticket's testcase and a crash/regression
with avxsynth (reported by Michael Niedermayer).

Also change frame->nb_samples < max to frame->nb_samples <= max.
This improves performance. Benchmark:
./ffmpeg -filter_complex "aevalsrc=0:n=1166,firequalizer=fixed=on" -f null null
old:
  25767 decicycles in take_samples,    1023 runs,      1 skips
  25422 decicycles in take_samples,    2047 runs,      1 skips
  25181 decicycles in take_samples,    4095 runs,      1 skips
  24904 decicycles in take_samples,    8191 runs,      1 skips

new:
    550 decicycles in take_samples,    1024 runs,      0 skips
    548 decicycles in take_samples,    2048 runs,      0 skips
    545 decicycles in take_samples,    4096 runs,      0 skips
    544 decicycles in take_samples,    8192 runs,      0 skips

Reviewed-by: Nicolas George <george@nsup.org>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>
