tools/target_dem_fuzzer: do not exit on io_buffer allocation failure

Failure to allocate a randomly sized buffer should not count as a anomaly in the fuzzer
there is nothing to fix in that case

Fixes: 490112574/clusterfuzz-testcase-minimized-ffmpeg_dem_TMV_fuzzer-6600485921685504
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2cce6ad03062b4cb10298513353ce55576ba7712)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>