ClusterFuzz marked crbug.com/oss-fuzz/66474 as Verified when the Fuzz Target did not exist

Please see https://crbug.com/oss-fuzz/66474#c6 and https://crbug.com/oss-fuzz/66474#c13.

Last week, for two days, there was a build failure in our UndefinedBehaviorSanitizer build so that the `build_fuzzers` command failed silengly (with exit status 0):

```
$ python infra/helper.py build_fuzzers --sanitizer undefined libavif ~/tmp/oss-fuzz-66755/libavif
...
[324/368] Building CXX object tests/CMakeF....dir/gtest/avif_fuzztest_enc_dec_anim.cc.o
ninja: build stopped: subcommand failed.
ls: cannot access './tests/avif_fuzztest_*': No such file or directory
...
```

but the `reproduce` command faild:

```
$ python infra/helper.py reproduce libavif avif_fuzztest_enc_dec_anim@EncodeDecodeAvifFuzzTest.EncodeDecodeAnimation ~/Downloads/clusterfuzz-testcase-minimized-avif_fuzztest_enc_dec_anim\@EncodeDecodeAvifFuzzTest.EncodeDecodeAnimation-4699314966298624
ERROR:__main__:avif_fuzztest_enc_dec_anim@EncodeDecodeAvifFuzzTest.EncodeDecodeAnimation does not seem to exist. Please run build_fuzzers first.
$ echo $?
1
$
```

Apparently this caused ClusterFuzz to mark crbug.com/oss-fuzz/66474 as Verified.