afl-fuzz: initial corpus is always created even if already exists

[fuzzah]

bb-fuzz checks if there's anything in the input dir, but the check is implemented improperly (in corpus_utils.py), as os.listdir() returns only names, but not relative paths.
This results in creating initial "corpus" every single time, even when it already exists.

It's a problem, because some fuzz targets don't run instrumented code if the sample is too short (i.e. partially instrumented apps such as .NET). And when such sample exists in the input dir, afl-fuzz can't start, as it detects no instrumentation on this one sample 🤷