libafl: fix lockfile ctr serialization (AFLplusplus#3319)

All locations reading from the lockfile expect an integer as a string,
but in `InMemoryOnDiskCorpus<I>::remove_testcase` the raw bytes of the
integer are written to the file in little endian byte order.

This may cause "ParseIntError { kind: InvalidDigit }" in e.g.
`InMemoryOnDiskCorpus<I>::save_testcase` during parsing of the file's
contents.

Fix this by writing the integer to the lockfile as a string in
`InMemoryOnDiskCorpus<I>::remove_testcase` as well.

Author: dergoegge

libafl/src/corpus/inmemory_ondisk.rs

@@ -490,7 +490,7 @@ impl<I> InMemoryOnDiskCorpus<I> {
                     drop(fs::remove_file(lockfile_path));
                 } else {
                     lockfile.seek(SeekFrom::Start(0))?;
-                    lockfile.write_all(&(ctr.parse::<u32>()? - 1).to_le_bytes())?;
+                    lockfile.write_all((ctr.parse::<u32>()? - 1).to_string().as_bytes())?;
                     return Ok(());
                 }
             }