Fix buffer overflow panics in frame metadata parsing

[podtserkovskiy]

Add bounds checks in trySeekMetadataLen, trySliceMetadata, and trySliceData
so malformed frames with a bogus metadata length return nil instead of
panicking on the slice expression.

[podtserkovskiy]

level=error msg="Running error: context loading failed: failed to load packages: failed to load packages: failed to load with go/packages: err: exit status 2: stderr: flag provided but not defined: -buildvcs\nusage: go list [-f format] [-json] [-m] [list flags] [build flags] [packages]\nRun 'go help list' for details.\n"

Lint error looks unrelated to the change

[jjeffcaii]

LGTM