tools/target_dem_fuzzer: Limit max blocks

tools/target_dem_fuzzer: Limit max blocks

With a IO block size of 1 byte potentially megabytes are quite slow to read, thus
limit the number

Fixes: 26511/clusterfuzz-testcase-minimized-ffmpeg_dem_NUV_fuzzer-5679249073373184
Fixes: 26517/clusterfuzz-testcase-minimized-ffmpeg_dem_XMV_fuzzer-6316634501021696
Fixes: 26518/clusterfuzz-testcase-minimized-ffmpeg_dem_WSVQA_fuzzer-485568285324083
Fixes: 26525/clusterfuzz-testcase-minimized-ffmpeg_dem_MSNWC_TCP_fuzzer-5121987011411968
Fixes: 26538/clusterfuzz-testcase-minimized-ffmpeg_dem_DHAV_fuzzer-5441800598454272
Fixes: OOM
Fixes: Timeout
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>