warc: Prevent signed integer overflow (#2568)

warc: Prevent signed integer overflow (#2568)

If a warc archive claims to have more than INT64_MAX - 4 content bytes,
the inevitable failure to skip all these bytes could lead to parsing
data which should be ignored instead.

The test case contains a conversation entry with that many bytes and if
the entry is not properly skipped, the warc implementation would read
the conversation data as a new file entry.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>