When reading a perf file (generated during a replay by mongor), the parser should be robust to malformed/empty files.

Specifically when parsing an empty file, failure to read enough bytes for the first field should be identified, rather than using an uninitialized field as a size, leading to bad allocs.

These are still exceptional cases, but should be identified and appropriate exceptions thrown.

Currently this is test-only functionality, no production use of mongor is affected.